/*
 *  linux/mm/memory.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 */

/*
 * demand-loading started 01.12.91 - seems it is high on the list of
 * things wanted, and it should be easy to implement. - Linus
 */

/*
 * Ok, demand-loading was easy, shared pages a little bit tricker. Shared
 * pages started 02.12.91, seems to work. - Linus.
 *
 * Tested sharing by executing about 30 /bin/sh: under the old kernel it
 * would have taken more than the 6M I have free, but it worked well as
 * far as I could see.
 *
 * Also corrected some "invalidate()"s - I wasn't doing enough of them.
 */

/*
 * Real VM (paging to/from disk) started 18.12.91. Much more work and
 * thought has to go into this. Oh, well..
 * 19.12.91  -  works, somewhat. Sometimes I get faults, don't know why.
 *              Found it. Everything seems to work now.
 * 20.12.91  -  Ok, making the swap-device changeable like the root.
 */

/*
 * 05.04.94  -  Multi-page memory management added for v1.1.
 *              Idea by Alex Bligh (alex@cconcepts.co.uk)
 *
 * 16.07.99  -  Support of BIGMEM added by Gerhard Wichert, Siemens AG
 *              (Gerhard.Wichert@pdb.siemens.de)
 *
 * Aug/Sep 2004 Changed to four level page tables (Andi Kleen)
 */

#include <linux/kernel_stat.h>
#include <linux/mm.h>
#include <linux/sched/mm.h>
#include <linux/sched/coredump.h>
#include <linux/sched/numa_balancing.h>
#include <linux/sched/task.h>
#include <linux/hugetlb.h>
#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/memremap.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/export.h>
#include <linux/delayacct.h>
#include <linux/init.h>
#include <linux/pfn_t.h>
#include <linux/writeback.h>
#include <linux/memcontrol.h>
#include <linux/mmu_notifier.h>
#include <linux/swapops.h>
#include <linux/elf.h>
#include <linux/gfp.h>
#include <linux/migrate.h>
#include <linux/string.h>
#include <linux/dma-debug.h>
#include <linux/debugfs.h>
#include <linux/userfaultfd_k.h>
#include <linux/dax.h>
#include <linux/oom.h>

#include <asm/io.h>
#include <asm/mmu_context.h>
#include <asm/pgalloc.h>
#include <linux/uaccess.h>
#include <asm/tlb.h>
#include <asm/tlbflush.h>
#include <asm/pgtable.h>

#include "internal.h"

#if defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS) && !defined(CONFIG_COMPILE_TEST)
#warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.
#endif

#ifndef CONFIG_NEED_MULTIPLE_NODES
/* use the per-pgdat data instead for discontigmem - mbligh */
unsigned long max_mapnr;
EXPORT_SYMBOL(max_mapnr);

struct page *mem_map;
EXPORT_SYMBOL(mem_map);
#endif

/*
 * A number of key systems in x86 including ioremap() rely on the assumption
 * that high_memory defines the upper bound on direct map memory, then end
 * of ZONE_NORMAL.  Under CONFIG_DISCONTIG this means that max_low_pfn and
 * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
 * and ZONE_HIGHMEM.
 */
void *high_memory;
EXPORT_SYMBOL(high_memory);

/*
 * Randomize the address space (stacks, mmaps, brk, etc.).
 *
 * ( When CONFIG_COMPAT_BRK=y we exclude brk from randomization,
 *   as ancient (libc5 based) binaries can segfault. )
 */
int randomize_va_space __read_mostly =
#ifdef CONFIG_COMPAT_BRK
                                        1;
#else
                                        2;
#endif

static int __init disable_randmaps(char *s)
{
        randomize_va_space = 0;
        return 1;
}
__setup("norandmaps", disable_randmaps);

unsigned long zero_pfn __read_mostly;
EXPORT_SYMBOL(zero_pfn);

unsigned long highest_memmap_pfn __read_mostly;

/*
 * CONFIG_MMU architectures set up ZERO_PAGE in their paging_init()
 */
static int __init init_zero_pfn(void)
{
        zero_pfn = page_to_pfn(ZERO_PAGE(0));
        return 0;
}
core_initcall(init_zero_pfn);


#if defined(SPLIT_RSS_COUNTING)

void sync_mm_rss(struct mm_struct *mm)
{
        int i;

        for (i = 0; i < NR_MM_COUNTERS; i++) {
                if (current->rss_stat.count[i]) {
                        add_mm_counter(mm, i, current->rss_stat.count[i]);
                        current->rss_stat.count[i] = 0;
                }
        }
        current->rss_stat.events = 0;
}

static void add_mm_counter_fast(struct mm_struct *mm, int member, int val)
{
        struct task_struct *task = current;

        if (likely(task->mm == mm))
                task->rss_stat.count[member] += val;
        else
                add_mm_counter(mm, member, val);
}
#define inc_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member, 1)
#define dec_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member, -1)

/* sync counter once per 64 page faults */
#define TASK_RSS_EVENTS_THRESH  (64)
static void check_sync_rss_stat(struct task_struct *task)
{
        if (unlikely(task != current))
                return;
        if (unlikely(task->rss_stat.events++ > TASK_RSS_EVENTS_THRESH))
                sync_mm_rss(task->mm);
}
#else /* SPLIT_RSS_COUNTING */

#define inc_mm_counter_fast(mm, member) inc_mm_counter(mm, member)
#define dec_mm_counter_fast(mm, member) dec_mm_counter(mm, member)

static void check_sync_rss_stat(struct task_struct *task)
{
}

#endif /* SPLIT_RSS_COUNTING */

/*
 * Note: this doesn't free the actual pages themselves. That
 * has been handled earlier when unmapping all the memory regions.
 */
static void free_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
                           unsigned long addr)
{
        pgtable_t token = pmd_pgtable(*pmd);
        pmd_clear(pmd);
        pte_free_tlb(tlb, token, addr);
        mm_dec_nr_ptes(tlb->mm);
}

static inline void free_pmd_range(struct mmu_gather *tlb, pud_t *pud,
                                unsigned long addr, unsigned long end,
                                unsigned long floor, unsigned long ceiling)
{
        pmd_t *pmd;
        unsigned long next;
        unsigned long start;

        start = addr;
        pmd = pmd_offset(pud, addr);
        do {
                next = pmd_addr_end(addr, end);
                if (pmd_none_or_clear_bad(pmd))
                        continue;
                free_pte_range(tlb, pmd, addr);
        } while (pmd++, addr = next, addr != end);

        start &= PUD_MASK;
        if (start < floor)
                return;
        if (ceiling) {
                ceiling &= PUD_MASK;
                if (!ceiling)
                        return;
        }
        if (end - 1 > ceiling - 1)
                return;

        pmd = pmd_offset(pud, start);
        pud_clear(pud);
        pmd_free_tlb(tlb, pmd, start);
        mm_dec_nr_pmds(tlb->mm);
}

static inline void free_pud_range(struct mmu_gather *tlb, p4d_t *p4d,
                                unsigned long addr, unsigned long end,
                                unsigned long floor, unsigned long ceiling)
{
        pud_t *pud;
        unsigned long next;
        unsigned long start;

        start = addr;
        pud = pud_offset(p4d, addr);
        do {
                next = pud_addr_end(addr, end);
                if (pud_none_or_clear_bad(pud))
                        continue;
                free_pmd_range(tlb, pud, addr, next, floor, ceiling);
        } while (pud++, addr = next, addr != end);

        start &= P4D_MASK;
        if (start < floor)
                return;
        if (ceiling) {
                ceiling &= P4D_MASK;
                if (!ceiling)
                        return;
        }
        if (end - 1 > ceiling - 1)
                return;

        pud = pud_offset(p4d, start);
        p4d_clear(p4d);
        pud_free_tlb(tlb, pud, start);
        mm_dec_nr_puds(tlb->mm);
}

static inline void free_p4d_range(struct mmu_gather *tlb, pgd_t *pgd,
                                unsigned long addr, unsigned long end,
                                unsigned long floor, unsigned long ceiling)
{
        p4d_t *p4d;
        unsigned long next;
        unsigned long start;

        start = addr;
        p4d = p4d_offset(pgd, addr);
        do {
                next = p4d_addr_end(addr, end);
                if (p4d_none_or_clear_bad(p4d))
                        continue;
                free_pud_range(tlb, p4d, addr, next, floor, ceiling);
        } while (p4d++, addr = next, addr != end);

        start &= PGDIR_MASK;
        if (start < floor)
                return;
        if (ceiling) {
                ceiling &= PGDIR_MASK;
                if (!ceiling)
                        return;
        }
        if (end - 1 > ceiling - 1)
                return;

        p4d = p4d_offset(pgd, start);
        pgd_clear(pgd);
        p4d_free_tlb(tlb, p4d, start);
}

/*
 * This function frees user-level page tables of a process.
 */
void free_pgd_range(struct mmu_gather *tlb,
                        unsigned long addr, unsigned long end,
                        unsigned long floor, unsigned long ceiling)
{
        pgd_t *pgd;
        unsigned long next;

        /*
         * The next few lines have given us lots of grief...
         *
         * Why are we testing PMD* at this top level?  Because often
         * there will be no work to do at all, and we'd prefer not to
         * go all the way down to the bottom just to discover that.
         *
         * Why all these "- 1"s?  Because 0 represents both the bottom
         * of the address space and the top of it (using -1 for the
         * top wouldn't help much: the masks would do the wrong thing).
         * The rule is that addr 0 and floor 0 refer to the bottom of
         * the address space, but end 0 and ceiling 0 refer to the top
         * Comparisons need to use "end - 1" and "ceiling - 1" (though
         * that end 0 case should be mythical).
         *
         * Wherever addr is brought up or ceiling brought down, we must
         * be careful to reject "the opposite 0" before it confuses the
         * subsequent tests.  But what about where end is brought down
         * by PMD_SIZE below? no, end can't go down to 0 there.
         *
         * Whereas we round start (addr) and ceiling down, by different
         * masks at different levels, in order to test whether a table
         * now has no other vmas using it, so can be freed, we don't
         * bother to round floor or end up - the tests don't need that.
         */

        addr &= PMD_MASK;
        if (addr < floor) {
                addr += PMD_SIZE;
                if (!addr)
                        return;
        }
        if (ceiling) {
                ceiling &= PMD_MASK;
                if (!ceiling)
                        return;
        }
        if (end - 1 > ceiling - 1)
                end -= PMD_SIZE;
        if (addr > end - 1)
                return;
        /*
         * We add page table cache pages with PAGE_SIZE,
         * (see pte_free_tlb()), flush the tlb if we need
         */
        tlb_remove_check_page_size_change(tlb, PAGE_SIZE);
        pgd = pgd_offset(tlb->mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                if (pgd_none_or_clear_bad(pgd))
                        continue;
                free_p4d_range(tlb, pgd, addr, next, floor, ceiling);
        } while (pgd++, addr = next, addr != end);
}

void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
                unsigned long floor, unsigned long ceiling)
{
        while (vma) {
                struct vm_area_struct *next = vma->vm_next;
                unsigned long addr = vma->vm_start;

                /*
                 * Hide vma from rmap and truncate_pagecache before freeing
                 * pgtables
                 */
                unlink_anon_vmas(vma);
                unlink_file_vma(vma);

                if (is_vm_hugetlb_page(vma)) {
                        hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
                                floor, next ? next->vm_start : ceiling);
                } else {
                        /*
                         * Optimization: gather nearby vmas into one call down
                         */
                        while (next && next->vm_start <= vma->vm_end + PMD_SIZE
                               && !is_vm_hugetlb_page(next)) {
                                vma = next;
                                next = vma->vm_next;
                                unlink_anon_vmas(vma);
                                unlink_file_vma(vma);
                        }
                        free_pgd_range(tlb, addr, vma->vm_end,
                                floor, next ? next->vm_start : ceiling);
                }
                vma = next;
        }
}

int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
{
        spinlock_t *ptl;
        pgtable_t new = pte_alloc_one(mm);
        if (!new)
                return -ENOMEM;

        /*
         * Ensure all pte setup (eg. pte page lock and page clearing) are
         * visible before the pte is made visible to other CPUs by being
         * put into page tables.
         *
         * The other side of the story is the pointer chasing in the page
         * table walking code (when walking the page table without locking;
         * ie. most of the time). Fortunately, these data accesses consist
         * of a chain of data-dependent loads, meaning most CPUs (alpha
         * being the notable exception) will already guarantee loads are
         * seen in-order. See the alpha page table accessors for the
         * smp_read_barrier_depends() barriers in page table walking code.
         */
        smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */

        ptl = pmd_lock(mm, pmd);
        if (likely(pmd_none(*pmd))) {   /* Has another populated it ? */
                mm_inc_nr_ptes(mm);
                pmd_populate(mm, pmd, new);
                new = NULL;
        }
        spin_unlock(ptl);
        if (new)
                pte_free(mm, new);
        return 0;
}

int __pte_alloc_kernel(pmd_t *pmd)
{
        pte_t *new = pte_alloc_one_kernel(&init_mm);
        if (!new)
                return -ENOMEM;

        smp_wmb(); /* See comment in __pte_alloc */

        spin_lock(&init_mm.page_table_lock);
        if (likely(pmd_none(*pmd))) {   /* Has another populated it ? */
                pmd_populate_kernel(&init_mm, pmd, new);
                new = NULL;
        }
        spin_unlock(&init_mm.page_table_lock);
        if (new)
                pte_free_kernel(&init_mm, new);
        return 0;
}

static inline void init_rss_vec(int *rss)
{
        memset(rss, 0, sizeof(int) * NR_MM_COUNTERS);
}

static inline void add_mm_rss_vec(struct mm_struct *mm, int *rss)
{
        int i;

        if (current->mm == mm)
                sync_mm_rss(mm);
        for (i = 0; i < NR_MM_COUNTERS; i++)
                if (rss[i])
                        add_mm_counter(mm, i, rss[i]);
}

/*
 * This function is called to print an error when a bad pte
 * is found. For example, we might have a PFN-mapped pte in
 * a region that doesn't allow it.
 *
 * The calling function must still handle the error.
 */
static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
                          pte_t pte, struct page *page)
{
        pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
        p4d_t *p4d = p4d_offset(pgd, addr);
        pud_t *pud = pud_offset(p4d, addr);
        pmd_t *pmd = pmd_offset(pud, addr);
        struct address_space *mapping;
        pgoff_t index;
        static unsigned long resume;
        static unsigned long nr_shown;
        static unsigned long nr_unshown;

        /*
         * Allow a burst of 60 reports, then keep quiet for that minute;
         * or allow a steady drip of one report per second.
         */
        if (nr_shown == 60) {
                if (time_before(jiffies, resume)) {
                        nr_unshown++;
                        return;
                }
                if (nr_unshown) {
                        pr_alert("BUG: Bad page map: %lu messages suppressed\n",
                                 nr_unshown);
                        nr_unshown = 0;
                }
                nr_shown = 0;
        }
        if (nr_shown++ == 0)
                resume = jiffies + 60 * HZ;

        mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
        index = linear_page_index(vma, addr);

        pr_alert("BUG: Bad page map in process %s  pte:%08llx pmd:%08llx\n",
                 current->comm,
                 (long long)pte_val(pte), (long long)pmd_val(*pmd));
        if (page)
                dump_page(page, "bad pte");
        pr_alert("addr:%p vm_flags:%08lx anon_vma:%p mapping:%p index:%lx\n",
                 (void *)addr, vma->vm_flags, vma->anon_vma, mapping, index);
        pr_alert("file:%pD fault:%pf mmap:%pf readpage:%pf\n",
                 vma->vm_file,
                 vma->vm_ops ? vma->vm_ops->fault : NULL,
                 vma->vm_file ? vma->vm_file->f_op->mmap : NULL,
                 mapping ? mapping->a_ops->readpage : NULL);
        dump_stack();
        add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
}

/*
 * vm_normal_page -- This function gets the "struct page" associated with a pte.
 *
 * "Special" mappings do not wish to be associated with a "struct page" (either
 * it doesn't exist, or it exists but they don't want to touch it). In this
 * case, NULL is returned here. "Normal" mappings do have a struct page.
 *
 * There are 2 broad cases. Firstly, an architecture may define a pte_special()
 * pte bit, in which case this function is trivial. Secondly, an architecture
 * may not have a spare pte bit, which requires a more complicated scheme,
 * described below.
 *
 * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
 * special mapping (even if there are underlying and valid "struct pages").
 * COWed pages of a VM_PFNMAP are always normal.
 *
 * The way we recognize COWed pages within VM_PFNMAP mappings is through the
 * rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit
 * set, and the vm_pgoff will point to the first PFN mapped: thus every special
 * mapping will always honor the rule
 *
 *      pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
 *
 * And for normal mappings this is false.
 *
 * This restricts such mappings to be a linear translation from virtual address
 * to pfn. To get around this restriction, we allow arbitrary mappings so long
 * as the vma is not a COW mapping; in that case, we know that all ptes are
 * special (because none can have been COWed).
 *
 *
 * In order to support COW of arbitrary special mappings, we have VM_MIXEDMAP.
 *
 * VM_MIXEDMAP mappings can likewise contain memory with or without "struct
 * page" backing, however the difference is that _all_ pages with a struct
 * page (that is, those where pfn_valid is true) are refcounted and considered
 * normal pages by the VM. The disadvantage is that pages are refcounted
 * (which can be slower and simply not an option for some PFNMAP users). The
 * advantage is that we don't have to follow the strict linearity rule of
 * PFNMAP mappings in order to support COWable mappings.
 *
 */
struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
                             pte_t pte, bool with_public_device)
{
        unsigned long pfn = pte_pfn(pte);

        if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
                if (likely(!pte_special(pte)))
                        goto check_pfn;
                if (vma->vm_ops && vma->vm_ops->find_special_page)
                        return vma->vm_ops->find_special_page(vma, addr);
                if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
                        return NULL;
                if (is_zero_pfn(pfn))
                        return NULL;

                /*
                 * Device public pages are special pages (they are ZONE_DEVICE
                 * pages but different from persistent memory). They behave
                 * allmost like normal pages. The difference is that they are
                 * not on the lru and thus should never be involve with any-
                 * thing that involve lru manipulation (mlock, numa balancing,
                 * ...).
                 *
                 * This is why we still want to return NULL for such page from
                 * vm_normal_page() so that we do not have to special case all
                 * call site of vm_normal_page().
                 */
                if (likely(pfn <= highest_memmap_pfn)) {
                        struct page *page = pfn_to_page(pfn);

                        if (is_device_public_page(page)) {
                                if (with_public_device)
                                        return page;
                                return NULL;
                        }
                }

                if (pte_devmap(pte))
                        return NULL;

                print_bad_pte(vma, addr, pte, NULL);
                return NULL;
        }

        /* !CONFIG_ARCH_HAS_PTE_SPECIAL case follows: */

        if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
                if (vma->vm_flags & VM_MIXEDMAP) {
                        if (!pfn_valid(pfn))
                                return NULL;
                        goto out;
                } else {
                        unsigned long off;
                        off = (addr - vma->vm_start) >> PAGE_SHIFT;
                        if (pfn == vma->vm_pgoff + off)
                                return NULL;
                        if (!is_cow_mapping(vma->vm_flags))
                                return NULL;
                }
        }

        if (is_zero_pfn(pfn))
                return NULL;

check_pfn:
        if (unlikely(pfn > highest_memmap_pfn)) {
                print_bad_pte(vma, addr, pte, NULL);
                return NULL;
        }

        /*
         * NOTE! We still have PageReserved() pages in the page tables.
         * eg. VDSO mappings can cause them to exist.
         */
out:
        return pfn_to_page(pfn);
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
                                pmd_t pmd)
{
        unsigned long pfn = pmd_pfn(pmd);

        /*
         * There is no pmd_special() but there may be special pmds, e.g.
         * in a direct-access (dax) mapping, so let's just replicate the
         * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
         */
        if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
                if (vma->vm_flags & VM_MIXEDMAP) {
                        if (!pfn_valid(pfn))
                                return NULL;
                        goto out;
                } else {
                        unsigned long off;
                        off = (addr - vma->vm_start) >> PAGE_SHIFT;
                        if (pfn == vma->vm_pgoff + off)
                                return NULL;
                        if (!is_cow_mapping(vma->vm_flags))
                                return NULL;
                }
        }

        if (pmd_devmap(pmd))
                return NULL;
        if (is_zero_pfn(pfn))
                return NULL;
        if (unlikely(pfn > highest_memmap_pfn))
                return NULL;

        /*
         * NOTE! We still have PageReserved() pages in the page tables.
         * eg. VDSO mappings can cause them to exist.
         */
out:
        return pfn_to_page(pfn);
}
#endif

/*
 * copy one vm_area from one task to the other. Assumes the page tables
 * already present in the new task to be cleared in the whole range
 * covered by this vma.
 */

static inline unsigned long
copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma,
                unsigned long addr, int *rss)
{
        unsigned long vm_flags = vma->vm_flags;
        pte_t pte = *src_pte;
        struct page *page;

        /* pte contains position in swap or file, so copy. */
        if (unlikely(!pte_present(pte))) {
                swp_entry_t entry = pte_to_swp_entry(pte);

                if (likely(!non_swap_entry(entry))) {
                        if (swap_duplicate(entry) < 0)
                                return entry.val;

                        /* make sure dst_mm is on swapoff's mmlist. */
                        if (unlikely(list_empty(&dst_mm->mmlist))) {
                                spin_lock(&mmlist_lock);
                                if (list_empty(&dst_mm->mmlist))
                                        list_add(&dst_mm->mmlist,
                                                        &src_mm->mmlist);
                                spin_unlock(&mmlist_lock);
                        }
                        rss[MM_SWAPENTS]++;
                } else if (is_migration_entry(entry)) {
                        page = migration_entry_to_page(entry);

                        rss[mm_counter(page)]++;

                        if (is_write_migration_entry(entry) &&
                                        is_cow_mapping(vm_flags)) {
                                /*
                                 * COW mappings require pages in both
                                 * parent and child to be set to read.
                                 */
                                make_migration_entry_read(&entry);
                                pte = swp_entry_to_pte(entry);
                                if (pte_swp_soft_dirty(*src_pte))
                                        pte = pte_swp_mksoft_dirty(pte);
                                set_pte_at(src_mm, addr, src_pte, pte);
                        }
                } else if (is_device_private_entry(entry)) {
                        page = device_private_entry_to_page(entry);

                        /*
                         * Update rss count even for unaddressable pages, as
                         * they should treated just like normal pages in this
                         * respect.
                         *
                         * We will likely want to have some new rss counters
                         * for unaddressable pages, at some point. But for now
                         * keep things as they are.
                         */
                        get_page(page);
                        rss[mm_counter(page)]++;
                        page_dup_rmap(page, false);

                        /*
                         * We do not preserve soft-dirty information, because so
                         * far, checkpoint/restore is the only feature that
                         * requires that. And checkpoint/restore does not work
                         * when a device driver is involved (you cannot easily
                         * save and restore device driver state).
                         */
                        if (is_write_device_private_entry(entry) &&
                            is_cow_mapping(vm_flags)) {
                                make_device_private_entry_read(&entry);
                                pte = swp_entry_to_pte(entry);
                                set_pte_at(src_mm, addr, src_pte, pte);
                        }
                }
                goto out_set_pte;
        }

        /*
         * If it's a COW mapping, write protect it both
         * in the parent and the child
         */
        if (is_cow_mapping(vm_flags) && pte_write(pte)) {
                ptep_set_wrprotect(src_mm, addr, src_pte);
                pte = pte_wrprotect(pte);
        }

        /*
         * If it's a shared mapping, mark it clean in
         * the child
         */
        if (vm_flags & VM_SHARED)
                pte = pte_mkclean(pte);
        pte = pte_mkold(pte);

        page = vm_normal_page(vma, addr, pte);
        if (page) {
                get_page(page);
                page_dup_rmap(page, false);
                rss[mm_counter(page)]++;
        } else if (pte_devmap(pte)) {
                page = pte_page(pte);

                /*
                 * Cache coherent device memory behave like regular page and
                 * not like persistent memory page. For more informations see
                 * MEMORY_DEVICE_CACHE_COHERENT in memory_hotplug.h
                 */
                if (is_device_public_page(page)) {
                        get_page(page);
                        page_dup_rmap(page, false);
                        rss[mm_counter(page)]++;
                }
        }

out_set_pte:
        set_pte_at(dst_mm, addr, dst_pte, pte);
        return 0;
}

static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                   pmd_t *dst_pmd, pmd_t *src_pmd, struct vm_area_struct *vma,
                   unsigned long addr, unsigned long end)
{
        pte_t *orig_src_pte, *orig_dst_pte;
        pte_t *src_pte, *dst_pte;
        spinlock_t *src_ptl, *dst_ptl;
        int progress = 0;
        int rss[NR_MM_COUNTERS];
        swp_entry_t entry = (swp_entry_t){0};

again:
        init_rss_vec(rss);

        dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);
        if (!dst_pte)
                return -ENOMEM;
        src_pte = pte_offset_map(src_pmd, addr);
        src_ptl = pte_lockptr(src_mm, src_pmd);
        spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
        orig_src_pte = src_pte;
        orig_dst_pte = dst_pte;
        arch_enter_lazy_mmu_mode();

        do {
                /*
                 * We are holding two locks at this point - either of them
                 * could generate latencies in another task on another CPU.
                 */
                if (progress >= 32) {
                        progress = 0;
                        if (need_resched() ||
                            spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
                                break;
                }
                if (pte_none(*src_pte)) {
                        progress++;
                        continue;
                }
                entry.val = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte,
                                                        vma, addr, rss);
                if (entry.val)
                        break;
                progress += 8;
        } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);

        arch_leave_lazy_mmu_mode();
        spin_unlock(src_ptl);
        pte_unmap(orig_src_pte);
        add_mm_rss_vec(dst_mm, rss);
        pte_unmap_unlock(orig_dst_pte, dst_ptl);
        cond_resched();

        if (entry.val) {
                if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
                        return -ENOMEM;
                progress = 0;
        }
        if (addr != end)
                goto again;
        return 0;
}

static inline int copy_pmd_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                pud_t *dst_pud, pud_t *src_pud, struct vm_area_struct *vma,
                unsigned long addr, unsigned long end)
{
        pmd_t *src_pmd, *dst_pmd;
        unsigned long next;

        dst_pmd = pmd_alloc(dst_mm, dst_pud, addr);
        if (!dst_pmd)
                return -ENOMEM;
        src_pmd = pmd_offset(src_pud, addr);
        do {
                next = pmd_addr_end(addr, end);
                if (is_swap_pmd(*src_pmd) || pmd_trans_huge(*src_pmd)
                        || pmd_devmap(*src_pmd)) {
                        int err;
                        VM_BUG_ON_VMA(next-addr != HPAGE_PMD_SIZE, vma);
                        err = copy_huge_pmd(dst_mm, src_mm,
                                            dst_pmd, src_pmd, addr, vma);
                        if (err == -ENOMEM)
                                return -ENOMEM;
                        if (!err)
                                continue;
                        /* fall through */
                }
                if (pmd_none_or_clear_bad(src_pmd))
                        continue;
                if (copy_pte_range(dst_mm, src_mm, dst_pmd, src_pmd,
                                                vma, addr, next))
                        return -ENOMEM;
        } while (dst_pmd++, src_pmd++, addr = next, addr != end);
        return 0;
}

static inline int copy_pud_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                p4d_t *dst_p4d, p4d_t *src_p4d, struct vm_area_struct *vma,
                unsigned long addr, unsigned long end)
{
        pud_t *src_pud, *dst_pud;
        unsigned long next;

        dst_pud = pud_alloc(dst_mm, dst_p4d, addr);
        if (!dst_pud)
                return -ENOMEM;
        src_pud = pud_offset(src_p4d, addr);
        do {
                next = pud_addr_end(addr, end);
                if (pud_trans_huge(*src_pud) || pud_devmap(*src_pud)) {
                        int err;

                        VM_BUG_ON_VMA(next-addr != HPAGE_PUD_SIZE, vma);
                        err = copy_huge_pud(dst_mm, src_mm,
                                            dst_pud, src_pud, addr, vma);
                        if (err == -ENOMEM)
                                return -ENOMEM;
                        if (!err)
                                continue;
                        /* fall through */
                }
                if (pud_none_or_clear_bad(src_pud))
                        continue;
                if (copy_pmd_range(dst_mm, src_mm, dst_pud, src_pud,
                                                vma, addr, next))
                        return -ENOMEM;
        } while (dst_pud++, src_pud++, addr = next, addr != end);
        return 0;
}

static inline int copy_p4d_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                pgd_t *dst_pgd, pgd_t *src_pgd, struct vm_area_struct *vma,
                unsigned long addr, unsigned long end)
{
        p4d_t *src_p4d, *dst_p4d;
        unsigned long next;

        dst_p4d = p4d_alloc(dst_mm, dst_pgd, addr);
        if (!dst_p4d)
                return -ENOMEM;
        src_p4d = p4d_offset(src_pgd, addr);
        do {
                next = p4d_addr_end(addr, end);
                if (p4d_none_or_clear_bad(src_p4d))
                        continue;
                if (copy_pud_range(dst_mm, src_mm, dst_p4d, src_p4d,
                                                vma, addr, next))
                        return -ENOMEM;
        } while (dst_p4d++, src_p4d++, addr = next, addr != end);
        return 0;
}

int copy_page_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                struct vm_area_struct *vma)
{
        pgd_t *src_pgd, *dst_pgd;
        unsigned long next;
        unsigned long addr = vma->vm_start;
        unsigned long end = vma->vm_end;
        struct mmu_notifier_range range;
        bool is_cow;
        int ret;

        /*
         * Don't copy ptes where a page fault will fill them correctly.
         * Fork becomes much lighter when there are big shared or private
         * readonly mappings. The tradeoff is that copy_page_range is more
         * efficient than faulting.
         */
        if (!(vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) &&
                        !vma->anon_vma)
                return 0;

        if (is_vm_hugetlb_page(vma))
                return copy_hugetlb_page_range(dst_mm, src_mm, vma);

        if (unlikely(vma->vm_flags & VM_PFNMAP)) {
                /*
                 * We do not free on error cases below as remove_vma
                 * gets called on error from higher level routine
                 */
                ret = track_pfn_copy(vma);
                if (ret)
                        return ret;

        }

        /*
         * We need to invalidate the secondary MMU mappings only when
         * there could be a permission downgrade on the ptes of the
         * parent mm. And a permission downgrade will only happen if
         * is_cow_mapping() returns true.
         */
        is_cow = is_cow_mapping(vma->vm_flags);

        if (is_cow) {
                mmu_notifier_range_init(&range, src_mm, addr, end);
                mmu_notifier_invalidate_range_start(&range);
        }

        ret = 0;
        dst_pgd = pgd_offset(dst_mm, addr);
        src_pgd = pgd_offset(src_mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                if (pgd_none_or_clear_bad(src_pgd))
                        continue;
                if (unlikely(copy_p4d_range(dst_mm, src_mm, dst_pgd, src_pgd,
                                            vma, addr, next))) {
                        ret = -ENOMEM;
                        break;
                }
        } while (dst_pgd++, src_pgd++, addr = next, addr != end);

        if (is_cow)
                mmu_notifier_invalidate_range_end(&range);
        return ret;
}

static unsigned long zap_pte_range(struct mmu_gather *tlb,
                                struct vm_area_struct *vma, pmd_t *pmd,
                                unsigned long addr, unsigned long end,
                                struct zap_details *details)
{
        struct mm_struct *mm = tlb->mm;
        int force_flush = 0;
        int rss[NR_MM_COUNTERS];
        spinlock_t *ptl;
        pte_t *start_pte;
        pte_t *pte;
        swp_entry_t entry;

        tlb_remove_check_page_size_change(tlb, PAGE_SIZE);
again:
        init_rss_vec(rss);
        start_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
        pte = start_pte;
        flush_tlb_batched_pending(mm);
        arch_enter_lazy_mmu_mode();
        do {
                pte_t ptent = *pte;
                if (pte_none(ptent))
                        continue;

                if (pte_present(ptent)) {
                        struct page *page;

                        page = _vm_normal_page(vma, addr, ptent, true);
                        if (unlikely(details) && page) {
                                /*
                                 * unmap_shared_mapping_pages() wants to
                                 * invalidate cache without truncating:
                                 * unmap shared but keep private pages.
                                 */
                                if (details->check_mapping &&
                                    details->check_mapping != page_rmapping(page))
                                        continue;
                        }
                        ptent = ptep_get_and_clear_full(mm, addr, pte,
                                                        tlb->fullmm);
                        tlb_remove_tlb_entry(tlb, pte, addr);
                        if (unlikely(!page))
                                continue;

                        if (!PageAnon(page)) {
                                if (pte_dirty(ptent)) {
                                        force_flush = 1;
                                        set_page_dirty(page);
                                }
                                if (pte_young(ptent) &&
                                    likely(!(vma->vm_flags & VM_SEQ_READ)))
                                        mark_page_accessed(page);
                        }
                        rss[mm_counter(page)]--;
                        page_remove_rmap(page, false);
                        if (unlikely(page_mapcount(page) < 0))
                                print_bad_pte(vma, addr, ptent, page);
                        if (unlikely(__tlb_remove_page(tlb, page))) {
                                force_flush = 1;
                                addr += PAGE_SIZE;
                                break;
                        }
                        continue;
                }

                entry = pte_to_swp_entry(ptent);
                if (non_swap_entry(entry) && is_device_private_entry(entry)) {
                        struct page *page = device_private_entry_to_page(entry);

                        if (unlikely(details && details->check_mapping)) {
                                /*
                                 * unmap_shared_mapping_pages() wants to
                                 * invalidate cache without truncating:
                                 * unmap shared but keep private pages.
                                 */
                                if (details->check_mapping !=
                                    page_rmapping(page))
                                        continue;
                        }

                        pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
                        rss[mm_counter(page)]--;
                        page_remove_rmap(page, false);
                        put_page(page);
                        continue;
                }

                /* If details->check_mapping, we leave swap entries. */
                if (unlikely(details))
                        continue;

                entry = pte_to_swp_entry(ptent);
                if (!non_swap_entry(entry))
                        rss[MM_SWAPENTS]--;
                else if (is_migration_entry(entry)) {
                        struct page *page;

                        page = migration_entry_to_page(entry);
                        rss[mm_counter(page)]--;
                }
                if (unlikely(!free_swap_and_cache(entry)))
                        print_bad_pte(vma, addr, ptent, NULL);
                pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
        } while (pte++, addr += PAGE_SIZE, addr != end);

        add_mm_rss_vec(mm, rss);
        arch_leave_lazy_mmu_mode();

        /* Do the actual TLB flush before dropping ptl */
        if (force_flush)
                tlb_flush_mmu_tlbonly(tlb);
        pte_unmap_unlock(start_pte, ptl);

        /*
         * If we forced a TLB flush (either due to running out of
         * batch buffers or because we needed to flush dirty TLB
         * entries before releasing the ptl), free the batched
         * memory too. Restart if we didn't do everything.
         */
        if (force_flush) {
                force_flush = 0;
                tlb_flush_mmu_free(tlb);
                if (addr != end)
                        goto again;
        }

        return addr;
}

static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
                                struct vm_area_struct *vma, pud_t *pud,
                                unsigned long addr, unsigned long end,
                                struct zap_details *details)
{
        pmd_t *pmd;
        unsigned long next;

        pmd = pmd_offset(pud, addr);
        do {
                next = pmd_addr_end(addr, end);
                if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
                        if (next - addr != HPAGE_PMD_SIZE)
                                __split_huge_pmd(vma, pmd, addr, false, NULL);
                        else if (zap_huge_pmd(tlb, vma, pmd, addr))
                                goto next;
                        /* fall through */
                }
                /*
                 * Here there can be other concurrent MADV_DONTNEED or
                 * trans huge page faults running, and if the pmd is
                 * none or trans huge it can change under us. This is
                 * because MADV_DONTNEED holds the mmap_sem in read
                 * mode.
                 */
                if (pmd_none_or_trans_huge_or_clear_bad(pmd))
                        goto next;
                next = zap_pte_range(tlb, vma, pmd, addr, next, details);
next:
                cond_resched();
        } while (pmd++, addr = next, addr != end);

        return addr;
}

static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
                                struct vm_area_struct *vma, p4d_t *p4d,
                                unsigned long addr, unsigned long end,
                                struct zap_details *details)
{
        pud_t *pud;
        unsigned long next;

        pud = pud_offset(p4d, addr);
        do {
                next = pud_addr_end(addr, end);
                if (pud_trans_huge(*pud) || pud_devmap(*pud)) {
                        if (next - addr != HPAGE_PUD_SIZE) {
                                VM_BUG_ON_VMA(!rwsem_is_locked(&tlb->mm->mmap_sem), vma);
                                split_huge_pud(vma, pud, addr);
                        } else if (zap_huge_pud(tlb, vma, pud, addr))
                                goto next;
                        /* fall through */
                }
                if (pud_none_or_clear_bad(pud))
                        continue;
                next = zap_pmd_range(tlb, vma, pud, addr, next, details);
next:
                cond_resched();
        } while (pud++, addr = next, addr != end);

        return addr;
}

static inline unsigned long zap_p4d_range(struct mmu_gather *tlb,
                                struct vm_area_struct *vma, pgd_t *pgd,
                                unsigned long addr, unsigned long end,
                                struct zap_details *details)
{
        p4d_t *p4d;
        unsigned long next;

        p4d = p4d_offset(pgd, addr);
        do {
                next = p4d_addr_end(addr, end);
                if (p4d_none_or_clear_bad(p4d))
                        continue;
                next = zap_pud_range(tlb, vma, p4d, addr, next, details);
        } while (p4d++, addr = next, addr != end);

        return addr;
}

void unmap_page_range(struct mmu_gather *tlb,
                             struct vm_area_struct *vma,
                             unsigned long addr, unsigned long end,
                             struct zap_details *details)
{
        pgd_t *pgd;
        unsigned long next;

        BUG_ON(addr >= end);
        tlb_start_vma(tlb, vma);
        pgd = pgd_offset(vma->vm_mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                if (pgd_none_or_clear_bad(pgd))
                        continue;
                next = zap_p4d_range(tlb, vma, pgd, addr, next, details);
        } while (pgd++, addr = next, addr != end);
        tlb_end_vma(tlb, vma);
}


static void unmap_single_vma(struct mmu_gather *tlb,
                struct vm_area_struct *vma, unsigned long start_addr,
                unsigned long end_addr,
                struct zap_details *details)
{
        unsigned long start = max(vma->vm_start, start_addr);
        unsigned long end;

        if (start >= vma->vm_end)
                return;
        end = min(vma->vm_end, end_addr);
        if (end <= vma->vm_start)
                return;

        if (vma->vm_file)
                uprobe_munmap(vma, start, end);

        if (unlikely(vma->vm_flags & VM_PFNMAP))
                untrack_pfn(vma, 0, 0);

        if (start != end) {
                if (unlikely(is_vm_hugetlb_page(vma))) {
                        /*
                         * It is undesirable to test vma->vm_file as it
                         * should be non-null for valid hugetlb area.
                         * However, vm_file will be NULL in the error
                         * cleanup path of mmap_region. When
                         * hugetlbfs ->mmap method fails,
                         * mmap_region() nullifies vma->vm_file
                         * before calling this function to clean up.
                         * Since no pte has actually been setup, it is
                         * safe to do nothing in this case.
                         */
                        if (vma->vm_file) {
                                i_mmap_lock_write(vma->vm_file->f_mapping);
                                __unmap_hugepage_range_final(tlb, vma, start, end, NULL);
                                i_mmap_unlock_write(vma->vm_file->f_mapping);
                        }
                } else
                        unmap_page_range(tlb, vma, start, end, details);
        }
}

/**
 * unmap_vmas - unmap a range of memory covered by a list of vma's
 * @tlb: address of the caller's struct mmu_gather
 * @vma: the starting vma
 * @start_addr: virtual address at which to start unmapping
 * @end_addr: virtual address at which to end unmapping
 *
 * Unmap all pages in the vma list.
 *
 * Only addresses between `start' and `end' will be unmapped.
 *
 * The VMA list must be sorted in ascending virtual address order.
 *
 * unmap_vmas() assumes that the caller will flush the whole unmapped address
 * range after unmap_vmas() returns.  So the only responsibility here is to
 * ensure that any thus-far unmapped pages are flushed before unmap_vmas()
 * drops the lock and schedules.
 */
void unmap_vmas(struct mmu_gather *tlb,
                struct vm_area_struct *vma, unsigned long start_addr,
                unsigned long end_addr)
{
        struct mmu_notifier_range range;

        mmu_notifier_range_init(&range, vma->vm_mm, start_addr, end_addr);
        mmu_notifier_invalidate_range_start(&range);
        for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next)
                unmap_single_vma(tlb, vma, start_addr, end_addr, NULL);
        mmu_notifier_invalidate_range_end(&range);
}

/**
 * zap_page_range - remove user pages in a given range
 * @vma: vm_area_struct holding the applicable pages
 * @start: starting address of pages to zap
 * @size: number of bytes to zap
 *
 * Caller must protect the VMA list
 */
void zap_page_range(struct vm_area_struct *vma, unsigned long start,
                unsigned long size)
{
        struct mmu_notifier_range range;
        struct mmu_gather tlb;

        lru_add_drain();
        mmu_notifier_range_init(&range, vma->vm_mm, start, start + size);
        tlb_gather_mmu(&tlb, vma->vm_mm, start, range.end);
        update_hiwater_rss(vma->vm_mm);
        mmu_notifier_invalidate_range_start(&range);
        for ( ; vma && vma->vm_start < range.end; vma = vma->vm_next)
                unmap_single_vma(&tlb, vma, start, range.end, NULL);
        mmu_notifier_invalidate_range_end(&range);
        tlb_finish_mmu(&tlb, start, range.end);
}

/**
 * zap_page_range_single - remove user pages in a given range
 * @vma: vm_area_struct holding the applicable pages
 * @address: starting address of pages to zap
 * @size: number of bytes to zap
 * @details: details of shared cache invalidation
 *
 * The range must fit into one VMA.
 */
static void zap_page_range_single(struct vm_area_struct *vma, unsigned long address,
                unsigned long size, struct zap_details *details)
{
        struct mmu_notifier_range range;
        struct mmu_gather tlb;

        lru_add_drain();
        mmu_notifier_range_init(&range, vma->vm_mm, address, address + size);
        tlb_gather_mmu(&tlb, vma->vm_mm, address, range.end);
        update_hiwater_rss(vma->vm_mm);
        mmu_notifier_invalidate_range_start(&range);
        unmap_single_vma(&tlb, vma, address, range.end, details);
        mmu_notifier_invalidate_range_end(&range);
        tlb_finish_mmu(&tlb, address, range.end);
}

/**
 * zap_vma_ptes - remove ptes mapping the vma
 * @vma: vm_area_struct holding ptes to be zapped
 * @address: starting address of pages to zap
 * @size: number of bytes to zap
 *
 * This function only unmaps ptes assigned to VM_PFNMAP vmas.
 *
 * The entire address range must be fully contained within the vma.
 *
 */
void zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
                unsigned long size)
{
        if (address < vma->vm_start || address + size > vma->vm_end ||
                        !(vma->vm_flags & VM_PFNMAP))
                return;

        zap_page_range_single(vma, address, size, NULL);
}
EXPORT_SYMBOL_GPL(zap_vma_ptes);

pte_t *__get_locked_pte(struct mm_struct *mm, unsigned long addr,
                        spinlock_t **ptl)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;

        pgd = pgd_offset(mm, addr);
        p4d = p4d_alloc(mm, pgd, addr);
        if (!p4d)
                return NULL;
        pud = pud_alloc(mm, p4d, addr);
        if (!pud)
                return NULL;
        pmd = pmd_alloc(mm, pud, addr);
        if (!pmd)
                return NULL;

        VM_BUG_ON(pmd_trans_huge(*pmd));
        return pte_alloc_map_lock(mm, pmd, addr, ptl);
}

/*
 * This is the old fallback for page remapping.
 *
 * For historical reasons, it only allows reserved pages. Only
 * old drivers should use this, and they needed to mark their
 * pages reserved for the old functions anyway.
 */
static int insert_page(struct vm_area_struct *vma, unsigned long addr,
                        struct page *page, pgprot_t prot)
{
        struct mm_struct *mm = vma->vm_mm;
        int retval;
        pte_t *pte;
        spinlock_t *ptl;

        retval = -EINVAL;
        if (PageAnon(page))
                goto out;
        retval = -ENOMEM;
        flush_dcache_page(page);
        pte = get_locked_pte(mm, addr, &ptl);
        if (!pte)
                goto out;
        retval = -EBUSY;
        if (!pte_none(*pte))
                goto out_unlock;

        /* Ok, finally just insert the thing.. */
        get_page(page);
        inc_mm_counter_fast(mm, mm_counter_file(page));
        page_add_file_rmap(page, false);
        set_pte_at(mm, addr, pte, mk_pte(page, prot));

        retval = 0;
        pte_unmap_unlock(pte, ptl);
        return retval;
out_unlock:
        pte_unmap_unlock(pte, ptl);
out:
        return retval;
}

/**
 * vm_insert_page - insert single page into user vma
 * @vma: user vma to map to
 * @addr: target user address of this page
 * @page: source kernel page
 *
 * This allows drivers to insert individual pages they've allocated
 * into a user vma.
 *
 * The page has to be a nice clean _individual_ kernel allocation.
 * If you allocate a compound page, you need to have marked it as
 * such (__GFP_COMP), or manually just split the page up yourself
 * (see split_page()).
 *
 * NOTE! Traditionally this was done with "remap_pfn_range()" which
 * took an arbitrary page protection parameter. This doesn't allow
 * that. Your vma protection will have to be set up correctly, which
 * means that if you want a shared writable mapping, you'd better
 * ask for a shared writable mapping!
 *
 * The page does not need to be reserved.
 *
 * Usually this function is called from f_op->mmap() handler
 * under mm->mmap_sem write-lock, so it can change vma->vm_flags.
 * Caller must set VM_MIXEDMAP on vma if it wants to call this
 * function from other places, for example from page-fault handler.
 */
int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
                        struct page *page)
{
        if (addr < vma->vm_start || addr >= vma->vm_end)
                return -EFAULT;
        if (!page_count(page))
                return -EINVAL;
        if (!(vma->vm_flags & VM_MIXEDMAP)) {
                BUG_ON(down_read_trylock(&vma->vm_mm->mmap_sem));
                BUG_ON(vma->vm_flags & VM_PFNMAP);
                vma->vm_flags |= VM_MIXEDMAP;
        }
        return insert_page(vma, addr, page, vma->vm_page_prot);
}
EXPORT_SYMBOL(vm_insert_page);

static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
                        pfn_t pfn, pgprot_t prot, bool mkwrite)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte, entry;
        spinlock_t *ptl;

        pte = get_locked_pte(mm, addr, &ptl);
        if (!pte)
                return VM_FAULT_OOM;
        if (!pte_none(*pte)) {
                if (mkwrite) {
                        /*
                         * For read faults on private mappings the PFN passed
                         * in may not match the PFN we have mapped if the
                         * mapped PFN is a writeable COW page.  In the mkwrite
                         * case we are creating a writable PTE for a shared
                         * mapping and we expect the PFNs to match. If they
                         * don't match, we are likely racing with block
                         * allocation and mapping invalidation so just skip the
                         * update.
                         */
                        if (pte_pfn(*pte) != pfn_t_to_pfn(pfn)) {
                                WARN_ON_ONCE(!is_zero_pfn(pte_pfn(*pte)));
                                goto out_unlock;
                        }
                        entry = *pte;
                        goto out_mkwrite;
                } else
                        goto out_unlock;
        }

        /* Ok, finally just insert the thing.. */
        if (pfn_t_devmap(pfn))
                entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
        else
                entry = pte_mkspecial(pfn_t_pte(pfn, prot));

out_mkwrite:
        if (mkwrite) {
                entry = pte_mkyoung(entry);
                entry = maybe_mkwrite(pte_mkdirty(entry), vma);
        }

        set_pte_at(mm, addr, pte, entry);
        update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */

out_unlock:
        pte_unmap_unlock(pte, ptl);
        return VM_FAULT_NOPAGE;
}

/**
 * vmf_insert_pfn_prot - insert single pfn into user vma with specified pgprot
 * @vma: user vma to map to
 * @addr: target user address of this page
 * @pfn: source kernel pfn
 * @pgprot: pgprot flags for the inserted page
 *
 * This is exactly like vmf_insert_pfn(), except that it allows drivers to
 * to override pgprot on a per-page basis.
 *
 * This only makes sense for IO mappings, and it makes no sense for
 * COW mappings.  In general, using multiple vmas is preferable;
 * vmf_insert_pfn_prot should only be used if using multiple VMAs is
 * impractical.
 *
 * Context: Process context.  May allocate using %GFP_KERNEL.
 * Return: vm_fault_t value.
 */
vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
                        unsigned long pfn, pgprot_t pgprot)
{
        /*
         * Technically, architectures with pte_special can avoid all these
         * restrictions (same for remap_pfn_range).  However we would like
         * consistency in testing and feature parity among all, so we should
         * try to keep these invariants in place for everybody.
         */
        BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
        BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
                                                (VM_PFNMAP|VM_MIXEDMAP));
        BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));
        BUG_ON((vma->vm_flags & VM_MIXEDMAP) && pfn_valid(pfn));

        if (addr < vma->vm_start || addr >= vma->vm_end)
                return VM_FAULT_SIGBUS;

        if (!pfn_modify_allowed(pfn, pgprot))
                return VM_FAULT_SIGBUS;

        track_pfn_insert(vma, &pgprot, __pfn_to_pfn_t(pfn, PFN_DEV));

        return insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot,
                        false);
}
EXPORT_SYMBOL(vmf_insert_pfn_prot);

/**
 * vmf_insert_pfn - insert single pfn into user vma
 * @vma: user vma to map to
 * @addr: target user address of this page
 * @pfn: source kernel pfn
 *
 * Similar to vm_insert_page, this allows drivers to insert individual pages
 * they've allocated into a user vma. Same comments apply.
 *
 * This function should only be called from a vm_ops->fault handler, and
 * in that case the handler should return the result of this function.
 *
 * vma cannot be a COW mapping.
 *
 * As this is called only for pages that do not currently exist, we
 * do not need to flush old virtual caches or the TLB.
 *
 * Context: Process context.  May allocate using %GFP_KERNEL.
 * Return: vm_fault_t value.
 */
vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
                        unsigned long pfn)
{
        return vmf_insert_pfn_prot(vma, addr, pfn, vma->vm_page_prot);
}
EXPORT_SYMBOL(vmf_insert_pfn);

static bool vm_mixed_ok(struct vm_area_struct *vma, pfn_t pfn)
{
        /* these checks mirror the abort conditions in vm_normal_page */
        if (vma->vm_flags & VM_MIXEDMAP)
                return true;
        if (pfn_t_devmap(pfn))
                return true;
        if (pfn_t_special(pfn))
                return true;
        if (is_zero_pfn(pfn_t_to_pfn(pfn)))
                return true;
        return false;
}

static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
                unsigned long addr, pfn_t pfn, bool mkwrite)
{
        pgprot_t pgprot = vma->vm_page_prot;
        int err;

        BUG_ON(!vm_mixed_ok(vma, pfn));

        if (addr < vma->vm_start || addr >= vma->vm_end)
                return VM_FAULT_SIGBUS;

        track_pfn_insert(vma, &pgprot, pfn);

        if (!pfn_modify_allowed(pfn_t_to_pfn(pfn), pgprot))
                return VM_FAULT_SIGBUS;

        /*
         * If we don't have pte special, then we have to use the pfn_valid()
         * based VM_MIXEDMAP scheme (see vm_normal_page), and thus we *must*
         * refcount the page if pfn_valid is true (hence insert_page rather
         * than insert_pfn).  If a zero_pfn were inserted into a VM_MIXEDMAP
         * without pte special, it would there be refcounted as a normal page.
         */
        if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) &&
            !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
                struct page *page;

                /*
                 * At this point we are committed to insert_page()
                 * regardless of whether the caller specified flags that
                 * result in pfn_t_has_page() == false.
                 */
                page = pfn_to_page(pfn_t_to_pfn(pfn));
                err = insert_page(vma, addr, page, pgprot);
        } else {
                return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
        }

        if (err == -ENOMEM)
                return VM_FAULT_OOM;
        if (err < 0 && err != -EBUSY)
                return VM_FAULT_SIGBUS;

        return VM_FAULT_NOPAGE;
}

vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
                pfn_t pfn)
{
        return __vm_insert_mixed(vma, addr, pfn, false);
}
EXPORT_SYMBOL(vmf_insert_mixed);

/*
 *  If the insertion of PTE failed because someone else already added a
 *  different entry in the mean time, we treat that as success as we assume
 *  the same entry was actually inserted.
 */
vm_fault_t vmf_insert_mixed_mkwrite(struct vm_area_struct *vma,
                unsigned long addr, pfn_t pfn)
{
        return __vm_insert_mixed(vma, addr, pfn, true);
}
EXPORT_SYMBOL(vmf_insert_mixed_mkwrite);

/*
 * maps a range of physical memory into the requested pages. the old
 * mappings are removed. any references to nonexistent pages results
 * in null mappings (currently treated as "copy-on-access")
 */
static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
                        unsigned long addr, unsigned long end,
                        unsigned long pfn, pgprot_t prot)
{
        pte_t *pte;
        spinlock_t *ptl;
        int err = 0;

        pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
        if (!pte)
                return -ENOMEM;
        arch_enter_lazy_mmu_mode();
        do {
                BUG_ON(!pte_none(*pte));
                if (!pfn_modify_allowed(pfn, prot)) {
                        err = -EACCES;
                        break;
                }
                set_pte_at(mm, addr, pte, pte_mkspecial(pfn_pte(pfn, prot)));
                pfn++;
        } while (pte++, addr += PAGE_SIZE, addr != end);
        arch_leave_lazy_mmu_mode();
        pte_unmap_unlock(pte - 1, ptl);
        return err;
}

static inline int remap_pmd_range(struct mm_struct *mm, pud_t *pud,
                        unsigned long addr, unsigned long end,
                        unsigned long pfn, pgprot_t prot)
{
        pmd_t *pmd;
        unsigned long next;
        int err;

        pfn -= addr >> PAGE_SHIFT;
        pmd = pmd_alloc(mm, pud, addr);
        if (!pmd)
                return -ENOMEM;
        VM_BUG_ON(pmd_trans_huge(*pmd));
        do {
                next = pmd_addr_end(addr, end);
                err = remap_pte_range(mm, pmd, addr, next,
                                pfn + (addr >> PAGE_SHIFT), prot);
                if (err)
                        return err;
        } while (pmd++, addr = next, addr != end);
        return 0;
}

static inline int remap_pud_range(struct mm_struct *mm, p4d_t *p4d,
                        unsigned long addr, unsigned long end,
                        unsigned long pfn, pgprot_t prot)
{
        pud_t *pud;
        unsigned long next;
        int err;

        pfn -= addr >> PAGE_SHIFT;
        pud = pud_alloc(mm, p4d, addr);
        if (!pud)
                return -ENOMEM;
        do {
                next = pud_addr_end(addr, end);
                err = remap_pmd_range(mm, pud, addr, next,
                                pfn + (addr >> PAGE_SHIFT), prot);
                if (err)
                        return err;
        } while (pud++, addr = next, addr != end);
        return 0;
}

static inline int remap_p4d_range(struct mm_struct *mm, pgd_t *pgd,
                        unsigned long addr, unsigned long end,
                        unsigned long pfn, pgprot_t prot)
{
        p4d_t *p4d;
        unsigned long next;
        int err;

        pfn -= addr >> PAGE_SHIFT;
        p4d = p4d_alloc(mm, pgd, addr);
        if (!p4d)
                return -ENOMEM;
        do {
                next = p4d_addr_end(addr, end);
                err = remap_pud_range(mm, p4d, addr, next,
                                pfn + (addr >> PAGE_SHIFT), prot);
                if (err)
                        return err;
        } while (p4d++, addr = next, addr != end);
        return 0;
}

/**
 * remap_pfn_range - remap kernel memory to userspace
 * @vma: user vma to map to
 * @addr: target user address to start at
 * @pfn: physical address of kernel memory
 * @size: size of map area
 * @prot: page protection flags for this mapping
 *
 *  Note: this is only safe if the mm semaphore is held when called.
 */
int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
                    unsigned long pfn, unsigned long size, pgprot_t prot)
{
        pgd_t *pgd;
        unsigned long next;
        unsigned long end = addr + PAGE_ALIGN(size);
        struct mm_struct *mm = vma->vm_mm;
        unsigned long remap_pfn = pfn;
        int err;

        /*
         * Physically remapped pages are special. Tell the
         * rest of the world about it:
         *   VM_IO tells people not to look at these pages
         *      (accesses can have side effects).
         *   VM_PFNMAP tells the core MM that the base pages are just
         *      raw PFN mappings, and do not have a "struct page" associated
         *      with them.
         *   VM_DONTEXPAND
         *      Disable vma merging and expanding with mremap().
         *   VM_DONTDUMP
         *      Omit vma from core dump, even when VM_IO turned off.
         *
         * There's a horrible special case to handle copy-on-write
         * behaviour that some programs depend on. We mark the "original"
         * un-COW'ed pages by matching them up with "vma->vm_pgoff".
         * See vm_normal_page() for details.
         */
        if (is_cow_mapping(vma->vm_flags)) {
                if (addr != vma->vm_start || end != vma->vm_end)
                        return -EINVAL;
                vma->vm_pgoff = pfn;
        }

        err = track_pfn_remap(vma, &prot, remap_pfn, addr, PAGE_ALIGN(size));
        if (err)
                return -EINVAL;

        vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;

        BUG_ON(addr >= end);
        pfn -= addr >> PAGE_SHIFT;
        pgd = pgd_offset(mm, addr);
        flush_cache_range(vma, addr, end);
        do {
                next = pgd_addr_end(addr, end);
                err = remap_p4d_range(mm, pgd, addr, next,
                                pfn + (addr >> PAGE_SHIFT), prot);
                if (err)
                        break;
        } while (pgd++, addr = next, addr != end);

        if (err)
                untrack_pfn(vma, remap_pfn, PAGE_ALIGN(size));

        return err;
}
EXPORT_SYMBOL(remap_pfn_range);

/**
 * vm_iomap_memory - remap memory to userspace
 * @vma: user vma to map to
 * @start: start of area
 * @len: size of area
 *
 * This is a simplified io_remap_pfn_range() for common driver use. The
 * driver just needs to give us the physical memory range to be mapped,
 * we'll figure out the rest from the vma information.
 *
 * NOTE! Some drivers might want to tweak vma->vm_page_prot first to get
 * whatever write-combining details or similar.
 */
int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len)
{
        unsigned long vm_len, pfn, pages;

        /* Check that the physical memory area passed in looks valid */
        if (start + len < start)
                return -EINVAL;
        /*
         * You *really* shouldn't map things that aren't page-aligned,
         * but we've historically allowed it because IO memory might
         * just have smaller alignment.
         */
        len += start & ~PAGE_MASK;
        pfn = start >> PAGE_SHIFT;
        pages = (len + ~PAGE_MASK) >> PAGE_SHIFT;
        if (pfn + pages < pfn)
                return -EINVAL;

        /* We start the mapping 'vm_pgoff' pages into the area */
        if (vma->vm_pgoff > pages)
                return -EINVAL;
        pfn += vma->vm_pgoff;
        pages -= vma->vm_pgoff;

        /* Can we fit all of the mapping? */
        vm_len = vma->vm_end - vma->vm_start;
        if (vm_len >> PAGE_SHIFT > pages)
                return -EINVAL;

        /* Ok, let it rip */
        return io_remap_pfn_range(vma, vma->vm_start, pfn, vm_len, vma->vm_page_prot);
}
EXPORT_SYMBOL(vm_iomap_memory);

static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
                                     unsigned long addr, unsigned long end,
                                     pte_fn_t fn, void *data)
{
        pte_t *pte;
        int err;
        pgtable_t token;
        spinlock_t *uninitialized_var(ptl);

        pte = (mm == &init_mm) ?
                pte_alloc_kernel(pmd, addr) :
                pte_alloc_map_lock(mm, pmd, addr, &ptl);
        if (!pte)
                return -ENOMEM;

        BUG_ON(pmd_huge(*pmd));

        arch_enter_lazy_mmu_mode();

        token = pmd_pgtable(*pmd);

        do {
                err = fn(pte++, token, addr, data);
                if (err)
                        break;
        } while (addr += PAGE_SIZE, addr != end);

        arch_leave_lazy_mmu_mode();

        if (mm != &init_mm)
                pte_unmap_unlock(pte-1, ptl);
        return err;
}

static int apply_to_pmd_range(struct mm_struct *mm, pud_t *pud,
                                     unsigned long addr, unsigned long end,
                                     pte_fn_t fn, void *data)
{
        pmd_t *pmd;
        unsigned long next;
        int err;

        BUG_ON(pud_huge(*pud));

        pmd = pmd_alloc(mm, pud, addr);
        if (!pmd)
                return -ENOMEM;
        do {
                next = pmd_addr_end(addr, end);
                err = apply_to_pte_range(mm, pmd, addr, next, fn, data);
                if (err)
                        break;
        } while (pmd++, addr = next, addr != end);
        return err;
}

static int apply_to_pud_range(struct mm_struct *mm, p4d_t *p4d,
                                     unsigned long addr, unsigned long end,
                                     pte_fn_t fn, void *data)
{

        pud_t *pud;
        unsigned long next;
        int err;

        pud = pud_alloc(mm, p4d, addr);
        if (!pud)
                return -ENOMEM;
        do {
                next = pud_addr_end(addr, end);
                err = apply_to_pmd_range(mm, pud, addr, next, fn, data);
                if (err)
                        break;
        } while (pud++, addr = next, addr != end);
        return err;
}

static int apply_to_p4d_range(struct mm_struct *mm, pgd_t *pgd,
                                     unsigned long addr, unsigned long end,
                                     pte_fn_t fn, void *data)
{
        p4d_t *p4d;
        unsigned long next;
        int err;

        p4d = p4d_alloc(mm, pgd, addr);
        if (!p4d)
                return -ENOMEM;
        do {
                next = p4d_addr_end(addr, end);
                err = apply_to_pud_range(mm, p4d, addr, next, fn, data);
                if (err)
                        break;
        } while (p4d++, addr = next, addr != end);
        return err;
}

/*
 * Scan a region of virtual memory, filling in page tables as necessary
 * and calling a provided function on each leaf page table.
 */
int apply_to_page_range(struct mm_struct *mm, unsigned long addr,
                        unsigned long size, pte_fn_t fn, void *data)
{
        pgd_t *pgd;
        unsigned long next;
        unsigned long end = addr + size;
        int err;

        if (WARN_ON(addr >= end))
                return -EINVAL;

        pgd = pgd_offset(mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                err = apply_to_p4d_range(mm, pgd, addr, next, fn, data);
                if (err)
                        break;
        } while (pgd++, addr = next, addr != end);

        return err;
}
EXPORT_SYMBOL_GPL(apply_to_page_range);

/*
 * handle_pte_fault chooses page fault handler according to an entry which was
 * read non-atomically.  Before making any commitment, on those architectures
 * or configurations (e.g. i386 with PAE) which might give a mix of unmatched
 * parts, do_swap_page must check under lock before unmapping the pte and
 * proceeding (but do_wp_page is only called after already making such a check;
 * and do_anonymous_page can safely check later on).
 */
static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
                                pte_t *page_table, pte_t orig_pte)
{
        int same = 1;
#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
        if (sizeof(pte_t) > sizeof(unsigned long)) {
                spinlock_t *ptl = pte_lockptr(mm, pmd);
                spin_lock(ptl);
                same = pte_same(*page_table, orig_pte);
                spin_unlock(ptl);
        }
#endif
        pte_unmap(page_table);
        return same;
}

static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
{
        debug_dma_assert_idle(src);

        /*
         * If the source page was a PFN mapping, we don't have
         * a "struct page" for it. We do a best-effort copy by
         * just copying from the original user address. If that
         * fails, we just zero-fill it. Live with it.
         */
        if (unlikely(!src)) {
                void *kaddr = kmap_atomic(dst);
                void __user *uaddr = (void __user *)(va & PAGE_MASK);

                /*
                 * This really shouldn't fail, because the page is there
                 * in the page tables. But it might just be unreadable,
                 * in which case we just give up and fill the result with
                 * zeroes.
                 */
                if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
                        clear_page(kaddr);
                kunmap_atomic(kaddr);
                flush_dcache_page(dst);
        } else
                copy_user_highpage(dst, src, va, vma);
}

static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
{
        struct file *vm_file = vma->vm_file;

        if (vm_file)
                return mapping_gfp_mask(vm_file->f_mapping) | __GFP_FS | __GFP_IO;

        /*
         * Special mappings (e.g. VDSO) do not have any file so fake
         * a default GFP_KERNEL for them.
         */
        return GFP_KERNEL;
}

/*
 * Notify the address space that the page is about to become writable so that
 * it can prohibit this or wait for the page to get into an appropriate state.
 *
 * We do this without the lock held, so that it can sleep if it needs to.
 */
static vm_fault_t do_page_mkwrite(struct vm_fault *vmf)
{
        vm_fault_t ret;
        struct page *page = vmf->page;
        unsigned int old_flags = vmf->flags;

        vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;

        ret = vmf->vma->vm_ops->page_mkwrite(vmf);
        /* Restore original flags so that caller is not surprised */
        vmf->flags = old_flags;
        if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
                return ret;
        if (unlikely(!(ret & VM_FAULT_LOCKED))) {
                lock_page(page);
                if (!page->mapping) {
                        unlock_page(page);
                        return 0; /* retry */
                }
                ret |= VM_FAULT_LOCKED;
        } else
                VM_BUG_ON_PAGE(!PageLocked(page), page);
        return ret;
}

/*
 * Handle dirtying of a page in shared file mapping on a write fault.
 *
 * The function expects the page to be locked and unlocks it.
 */
static void fault_dirty_shared_page(struct vm_area_struct *vma,
                                    struct page *page)
{
        struct address_space *mapping;
        bool dirtied;
        bool page_mkwrite = vma->vm_ops && vma->vm_ops->page_mkwrite;

        dirtied = set_page_dirty(page);
        VM_BUG_ON_PAGE(PageAnon(page), page);
        /*
         * Take a local copy of the address_space - page.mapping may be zeroed
         * by truncate after unlock_page().   The address_space itself remains
         * pinned by vma->vm_file's reference.  We rely on unlock_page()'s
         * release semantics to prevent the compiler from undoing this copying.
         */
        mapping = page_rmapping(page);
        unlock_page(page);

        if ((dirtied || page_mkwrite) && mapping) {
                /*
                 * Some device drivers do not set page.mapping
                 * but still dirty their pages
                 */
                balance_dirty_pages_ratelimited(mapping);
        }

        if (!page_mkwrite)
                file_update_time(vma->vm_file);
}

/*
 * Handle write page faults for pages that can be reused in the current vma
 *
 * This can happen either due to the mapping being with the VM_SHARED flag,
 * or due to us being the last reference standing to the page. In either
 * case, all we need to do here is to mark the page as writable and update
 * any related book-keeping.
 */
static inline void wp_page_reuse(struct vm_fault *vmf)
        __releases(vmf->ptl)
{
        struct vm_area_struct *vma = vmf->vma;
        struct page *page = vmf->page;
        pte_t entry;
        /*
         * Clear the pages cpupid information as the existing
         * information potentially belongs to a now completely
         * unrelated process.
         */
        if (page)
                page_cpupid_xchg_last(page, (1 << LAST_CPUPID_SHIFT) - 1);

        flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
        entry = pte_mkyoung(vmf->orig_pte);
        entry = maybe_mkwrite(pte_mkdirty(entry), vma);
        if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
                update_mmu_cache(vma, vmf->address, vmf->pte);
        pte_unmap_unlock(vmf->pte, vmf->ptl);
}

/*
 * Handle the case of a page which we actually need to copy to a new page.
 *
 * Called with mmap_sem locked and the old page referenced, but
 * without the ptl held.
 *
 * High level logic flow:
 *
 * - Allocate a page, copy the content of the old page to the new one.
 * - Handle book keeping and accounting - cgroups, mmu-notifiers, etc.
 * - Take the PTL. If the pte changed, bail out and release the allocated page
 * - If the pte is still the way we remember it, update the page table and all
 *   relevant references. This includes dropping the reference the page-table
 *   held to the old page, as well as updating the rmap.
 * - In any case, unlock the PTL and drop the reference we took to the old page.
 */
static vm_fault_t wp_page_copy(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;
        struct mm_struct *mm = vma->vm_mm;
        struct page *old_page = vmf->page;
        struct page *new_page = NULL;
        pte_t entry;
        int page_copied = 0;
        struct mem_cgroup *memcg;
        struct mmu_notifier_range range;

        if (unlikely(anon_vma_prepare(vma)))
                goto oom;

        if (is_zero_pfn(pte_pfn(vmf->orig_pte))) {
                new_page = alloc_zeroed_user_highpage_movable(vma,
                                                              vmf->address);
                if (!new_page)
                        goto oom;
        } else {
                new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
                                vmf->address);
                if (!new_page)
                        goto oom;
                cow_user_page(new_page, old_page, vmf->address, vma);
        }

        if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))
                goto oom_free_new;

        __SetPageUptodate(new_page);

        mmu_notifier_range_init(&range, mm, vmf->address & PAGE_MASK,
                                (vmf->address & PAGE_MASK) + PAGE_SIZE);
        mmu_notifier_invalidate_range_start(&range);

        /*
         * Re-check the pte - we dropped the lock
         */
        vmf->pte = pte_offset_map_lock(mm, vmf->pmd, vmf->address, &vmf->ptl);
        if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
                if (old_page) {
                        if (!PageAnon(old_page)) {
                                dec_mm_counter_fast(mm,
                                                mm_counter_file(old_page));
                                inc_mm_counter_fast(mm, MM_ANONPAGES);
                        }
                } else {
                        inc_mm_counter_fast(mm, MM_ANONPAGES);
                }
                flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
                entry = mk_pte(new_page, vma->vm_page_prot);
                entry = maybe_mkwrite(pte_mkdirty(entry), vma);
                /*
                 * Clear the pte entry and flush it first, before updating the
                 * pte with the new entry. This will avoid a race condition
                 * seen in the presence of one thread doing SMC and another
                 * thread doing COW.
                 */
                ptep_clear_flush_notify(vma, vmf->address, vmf->pte);
                page_add_new_anon_rmap(new_page, vma, vmf->address, false);
                mem_cgroup_commit_charge(new_page, memcg, false, false);
                lru_cache_add_active_or_unevictable(new_page, vma);
                /*
                 * We call the notify macro here because, when using secondary
                 * mmu page tables (such as kvm shadow page tables), we want the
                 * new page to be mapped directly into the secondary page table.
                 */
                set_pte_at_notify(mm, vmf->address, vmf->pte, entry);
                update_mmu_cache(vma, vmf->address, vmf->pte);
                if (old_page) {
                        /*
                         * Only after switching the pte to the new page may
                         * we remove the mapcount here. Otherwise another
                         * process may come and find the rmap count decremented
                         * before the pte is switched to the new page, and
                         * "reuse" the old page writing into it while our pte
                         * here still points into it and can be read by other
                         * threads.
                         *
                         * The critical issue is to order this
                         * page_remove_rmap with the ptp_clear_flush above.
                         * Those stores are ordered by (if nothing else,)
                         * the barrier present in the atomic_add_negative
                         * in page_remove_rmap.
                         *
                         * Then the TLB flush in ptep_clear_flush ensures that
                         * no process can access the old page before the
                         * decremented mapcount is visible. And the old page
                         * cannot be reused until after the decremented
                         * mapcount is visible. So transitively, TLBs to
                         * old page will be flushed before it can be reused.
                         */
                        page_remove_rmap(old_page, false);
                }

                /* Free the old page.. */
                new_page = old_page;
                page_copied = 1;
        } else {
                mem_cgroup_cancel_charge(new_page, memcg, false);
        }

        if (new_page)
                put_page(new_page);

        pte_unmap_unlock(vmf->pte, vmf->ptl);
        /*
         * No need to double call mmu_notifier->invalidate_range() callback as
         * the above ptep_clear_flush_notify() did already call it.
         */
        mmu_notifier_invalidate_range_only_end(&range);
        if (old_page) {
                /*
                 * Don't let another task, with possibly unlocked vma,
                 * keep the mlocked page.
                 */
                if (page_copied && (vma->vm_flags & VM_LOCKED)) {
                        lock_page(old_page);    /* LRU manipulation */
                        if (PageMlocked(old_page))
                                munlock_vma_page(old_page);
                        unlock_page(old_page);
                }
                put_page(old_page);
        }
        return page_copied ? VM_FAULT_WRITE : 0;
oom_free_new:
        put_page(new_page);
oom:
        if (old_page)
                put_page(old_page);
        return VM_FAULT_OOM;
}

/**
 * finish_mkwrite_fault - finish page fault for a shared mapping, making PTE
 *                        writeable once the page is prepared
 *
 * @vmf: structure describing the fault
 *
 * This function handles all that is needed to finish a write page fault in a
 * shared mapping due to PTE being read-only once the mapped page is prepared.
 * It handles locking of PTE and modifying it. The function returns
 * VM_FAULT_WRITE on success, 0 when PTE got changed before we acquired PTE
 * lock.
 *
 * The function expects the page to be locked or other protection against
 * concurrent faults / writeback (such as DAX radix tree locks).
 */
vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf)
{
        WARN_ON_ONCE(!(vmf->vma->vm_flags & VM_SHARED));
        vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, vmf->address,
                                       &vmf->ptl);
        /*
         * We might have raced with another page fault while we released the
         * pte_offset_map_lock.
         */
        if (!pte_same(*vmf->pte, vmf->orig_pte)) {
                pte_unmap_unlock(vmf->pte, vmf->ptl);
                return VM_FAULT_NOPAGE;
        }
        wp_page_reuse(vmf);
        return 0;
}

/*
 * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
 * mapping
 */
static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;

        if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
                vm_fault_t ret;

                pte_unmap_unlock(vmf->pte, vmf->ptl);
                vmf->flags |= FAULT_FLAG_MKWRITE;
                ret = vma->vm_ops->pfn_mkwrite(vmf);
                if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
                        return ret;
                return finish_mkwrite_fault(vmf);
        }
        wp_page_reuse(vmf);
        return VM_FAULT_WRITE;
}

static vm_fault_t wp_page_shared(struct vm_fault *vmf)
        __releases(vmf->ptl)
{
        struct vm_area_struct *vma = vmf->vma;

        get_page(vmf->page);

        if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
                vm_fault_t tmp;

                pte_unmap_unlock(vmf->pte, vmf->ptl);
                tmp = do_page_mkwrite(vmf);
                if (unlikely(!tmp || (tmp &
                                      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
                        put_page(vmf->page);
                        return tmp;
                }
                tmp = finish_mkwrite_fault(vmf);
                if (unlikely(tmp & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) {
                        unlock_page(vmf->page);
                        put_page(vmf->page);
                        return tmp;
                }
        } else {
                wp_page_reuse(vmf);
                lock_page(vmf->page);
        }
        fault_dirty_shared_page(vma, vmf->page);
        put_page(vmf->page);

        return VM_FAULT_WRITE;
}

/*
 * This routine handles present pages, when users try to write
 * to a shared page. It is done by copying the page to a new address
 * and decrementing the shared-page counter for the old page.
 *
 * Note that this routine assumes that the protection checks have been
 * done by the caller (the low-level page fault routine in most cases).
 * Thus we can safely just mark it writable once we've done any necessary
 * COW.
 *
 * We also mark the page dirty at this point even though the page will
 * change only once the write actually happens. This avoids a few races,
 * and potentially makes it more efficient.
 *
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults), with pte both mapped and locked.
 * We return with mmap_sem still held, but pte unmapped and unlocked.
 */
static vm_fault_t do_wp_page(struct vm_fault *vmf)
        __releases(vmf->ptl)
{
        struct vm_area_struct *vma = vmf->vma;

        vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
        if (!vmf->page) {
                /*
                 * VM_MIXEDMAP !pfn_valid() case, or VM_SOFTDIRTY clear on a
                 * VM_PFNMAP VMA.
                 *
                 * We should not cow pages in a shared writeable mapping.
                 * Just mark the pages writable and/or call ops->pfn_mkwrite.
                 */
                if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
                                     (VM_WRITE|VM_SHARED))
                        return wp_pfn_shared(vmf);

                pte_unmap_unlock(vmf->pte, vmf->ptl);
                return wp_page_copy(vmf);
        }

        /*
         * Take out anonymous pages first, anonymous shared vmas are
         * not dirty accountable.
         */
        if (PageAnon(vmf->page) && !PageKsm(vmf->page)) {
                int total_map_swapcount;
                if (!trylock_page(vmf->page)) {
                        get_page(vmf->page);
                        pte_unmap_unlock(vmf->pte, vmf->ptl);
                        lock_page(vmf->page);
                        vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
                                        vmf->address, &vmf->ptl);
                        if (!pte_same(*vmf->pte, vmf->orig_pte)) {
                                unlock_page(vmf->page);
                                pte_unmap_unlock(vmf->pte, vmf->ptl);
                                put_page(vmf->page);
                                return 0;
                        }
                        put_page(vmf->page);
                }
                if (reuse_swap_page(vmf->page, &total_map_swapcount)) {
                        if (total_map_swapcount == 1) {
                                /*
                                 * The page is all ours. Move it to
                                 * our anon_vma so the rmap code will
                                 * not search our parent or siblings.
                                 * Protected against the rmap code by
                                 * the page lock.
                                 */
                                page_move_anon_rmap(vmf->page, vma);
                        }
                        unlock_page(vmf->page);
                        wp_page_reuse(vmf);
                        return VM_FAULT_WRITE;
                }
                unlock_page(vmf->page);
        } else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
                                        (VM_WRITE|VM_SHARED))) {
                return wp_page_shared(vmf);
        }

        /*
         * Ok, we need to copy. Oh, well..
         */
        get_page(vmf->page);

        pte_unmap_unlock(vmf->pte, vmf->ptl);
        return wp_page_copy(vmf);
}

static void unmap_mapping_range_vma(struct vm_area_struct *vma,
                unsigned long start_addr, unsigned long end_addr,
                struct zap_details *details)
{
        zap_page_range_single(vma, start_addr, end_addr - start_addr, details);
}

static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
                                            struct zap_details *details)
{
        struct vm_area_struct *vma;
        pgoff_t vba, vea, zba, zea;

        vma_interval_tree_foreach(vma, root,
                        details->first_index, details->last_index) {

                vba = vma->vm_pgoff;
                vea = vba + vma_pages(vma) - 1;
                zba = details->first_index;
                if (zba < vba)
                        zba = vba;
                zea = details->last_index;
                if (zea > vea)
                        zea = vea;

                unmap_mapping_range_vma(vma,
                        ((zba - vba) << PAGE_SHIFT) + vma->vm_start,
                        ((zea - vba + 1) << PAGE_SHIFT) + vma->vm_start,
                                details);
        }
}

/**
 * unmap_mapping_pages() - Unmap pages from processes.
 * @mapping: The address space containing pages to be unmapped.
 * @start: Index of first page to be unmapped.
 * @nr: Number of pages to be unmapped.  0 to unmap to end of file.
 * @even_cows: Whether to unmap even private COWed pages.
 *
 * Unmap the pages in this address space from any userspace process which
 * has them mmaped.  Generally, you want to remove COWed pages as well when
 * a file is being truncated, but not when invalidating pages from the page
 * cache.
 */
void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
                pgoff_t nr, bool even_cows)
{
        struct zap_details details = { };

        details.check_mapping = even_cows ? NULL : mapping;
        details.first_index = start;
        details.last_index = start + nr - 1;
        if (details.last_index < details.first_index)
                details.last_index = ULONG_MAX;

        i_mmap_lock_write(mapping);
        if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
                unmap_mapping_range_tree(&mapping->i_mmap, &details);
        i_mmap_unlock_write(mapping);
}

/**
 * unmap_mapping_range - unmap the portion of all mmaps in the specified
 * address_space corresponding to the specified byte range in the underlying
 * file.
 *
 * @mapping: the address space containing mmaps to be unmapped.
 * @holebegin: byte in first page to unmap, relative to the start of
 * the underlying file.  This will be rounded down to a PAGE_SIZE
 * boundary.  Note that this is different from truncate_pagecache(), which
 * must keep the partial page.  In contrast, we must get rid of
 * partial pages.
 * @holelen: size of prospective hole in bytes.  This will be rounded
 * up to a PAGE_SIZE boundary.  A holelen of zero truncates to the
 * end of the file.
 * @even_cows: 1 when truncating a file, unmap even private COWed pages;
 * but 0 when invalidating pagecache, don't throw away private data.
 */
void unmap_mapping_range(struct address_space *mapping,
                loff_t const holebegin, loff_t const holelen, int even_cows)
{
        pgoff_t hba = holebegin >> PAGE_SHIFT;
        pgoff_t hlen = (holelen + PAGE_SIZE - 1) >> PAGE_SHIFT;

        /* Check for overflow. */
        if (sizeof(holelen) > sizeof(hlen)) {
                long long holeend =
                        (holebegin + holelen + PAGE_SIZE - 1) >> PAGE_SHIFT;
                if (holeend & ~(long long)ULONG_MAX)
                        hlen = ULONG_MAX - hba + 1;
        }

        unmap_mapping_pages(mapping, hba, hlen, even_cows);
}
EXPORT_SYMBOL(unmap_mapping_range);

/*
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults), and pte mapped but not yet locked.
 * We return with pte unmapped and unlocked.
 *
 * We return with the mmap_sem locked or unlocked in the same cases
 * as does filemap_fault().
 */
vm_fault_t do_swap_page(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;
        struct page *page = NULL, *swapcache;
        struct mem_cgroup *memcg;
        swp_entry_t entry;
        pte_t pte;
        int locked;
        int exclusive = 0;
        vm_fault_t ret = 0;

        if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
                goto out;

        entry = pte_to_swp_entry(vmf->orig_pte);
        if (unlikely(non_swap_entry(entry))) {
                if (is_migration_entry(entry)) {
                        migration_entry_wait(vma->vm_mm, vmf->pmd,
                                             vmf->address);
                } else if (is_device_private_entry(entry)) {
                        /*
                         * For un-addressable device memory we call the pgmap
                         * fault handler callback. The callback must migrate
                         * the page back to some CPU accessible page.
                         */
                        ret = device_private_entry_fault(vma, vmf->address, entry,
                                                 vmf->flags, vmf->pmd);
                } else if (is_hwpoison_entry(entry)) {
                        ret = VM_FAULT_HWPOISON;
                } else {
                        print_bad_pte(vma, vmf->address, vmf->orig_pte, NULL);
                        ret = VM_FAULT_SIGBUS;
                }
                goto out;
        }


        delayacct_set_flag(DELAYACCT_PF_SWAPIN);
        page = lookup_swap_cache(entry, vma, vmf->address);
        swapcache = page;

        if (!page) {
                struct swap_info_struct *si = swp_swap_info(entry);

                if (si->flags & SWP_SYNCHRONOUS_IO &&
                                __swap_count(si, entry) == 1) {
                        /* skip swapcache */
                        page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
                                                        vmf->address);
                        if (page) {
                                __SetPageLocked(page);
                                __SetPageSwapBacked(page);
                                set_page_private(page, entry.val);
                                lru_cache_add_anon(page);
                                swap_readpage(page, true);
                        }
                } else {
                        page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
                                                vmf);
                        swapcache = page;
                }

                if (!page) {
                        /*
                         * Back out if somebody else faulted in this pte
                         * while we released the pte lock.
                         */
                        vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
                                        vmf->address, &vmf->ptl);
                        if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
                                ret = VM_FAULT_OOM;
                        delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
                        goto unlock;
                }

                /* Had to read the page from swap area: Major fault */
                ret = VM_FAULT_MAJOR;
                count_vm_event(PGMAJFAULT);
                count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
        } else if (PageHWPoison(page)) {
                /*
                 * hwpoisoned dirty swapcache pages are kept for killing
                 * owner processes (which may be unknown at hwpoison time)
                 */
                ret = VM_FAULT_HWPOISON;
                delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
                goto out_release;
        }

        locked = lock_page_or_retry(page, vma->vm_mm, vmf->flags);

        delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
        if (!locked) {
                ret |= VM_FAULT_RETRY;
                goto out_release;
        }

        /*
         * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
         * release the swapcache from under us.  The page pin, and pte_same
         * test below, are not enough to exclude that.  Even if it is still
         * swapcache, we need to check that the page's swap has not changed.
         */
        if (unlikely((!PageSwapCache(page) ||
                        page_private(page) != entry.val)) && swapcache)
                goto out_page;

        page = ksm_might_need_to_copy(page, vma, vmf->address);
        if (unlikely(!page)) {
                ret = VM_FAULT_OOM;
                page = swapcache;
                goto out_page;
        }

        if (mem_cgroup_try_charge_delay(page, vma->vm_mm, GFP_KERNEL,
                                        &memcg, false)) {
                ret = VM_FAULT_OOM;
                goto out_page;
        }

        /*
         * Back out if somebody else already faulted in this pte.
         */
        vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
                        &vmf->ptl);
        if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
                goto out_nomap;

        if (unlikely(!PageUptodate(page))) {
                ret = VM_FAULT_SIGBUS;
                goto out_nomap;
        }

        /*
         * The page isn't present yet, go ahead with the fault.
         *
         * Be careful about the sequence of operations here.
         * To get its accounting right, reuse_swap_page() must be called
         * while the page is counted on swap but not yet in mapcount i.e.
         * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
         * must be called after the swap_free(), or it will never succeed.
         */

        inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
        dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
        pte = mk_pte(page, vma->vm_page_prot);
        if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
                pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                vmf->flags &= ~FAULT_FLAG_WRITE;
                ret |= VM_FAULT_WRITE;
                exclusive = RMAP_EXCLUSIVE;
        }
        flush_icache_page(vma, page);
        if (pte_swp_soft_dirty(vmf->orig_pte))
                pte = pte_mksoft_dirty(pte);
        set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
        arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
        vmf->orig_pte = pte;

        /* ksm created a completely new copy */
        if (unlikely(page != swapcache && swapcache)) {
                page_add_new_anon_rmap(page, vma, vmf->address, false);
                mem_cgroup_commit_charge(page, memcg, false, false);
                lru_cache_add_active_or_unevictable(page, vma);
        } else {
                do_page_add_anon_rmap(page, vma, vmf->address, exclusive);
                mem_cgroup_commit_charge(page, memcg, true, false);
                activate_page(page);
        }

        swap_free(entry);
        if (mem_cgroup_swap_full(page) ||
            (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
                try_to_free_swap(page);
        unlock_page(page);
        if (page != swapcache && swapcache) {
                /*
                 * Hold the lock to avoid the swap entry to be reused
                 * until we take the PT lock for the pte_same() check
                 * (to avoid false positives from pte_same). For
                 * further safety release the lock after the swap_free
                 * so that the swap count won't change under a
                 * parallel locked swapcache.
                 */
                unlock_page(swapcache);
                put_page(swapcache);
        }

        if (vmf->flags & FAULT_FLAG_WRITE) {
                ret |= do_wp_page(vmf);
                if (ret & VM_FAULT_ERROR)
                        ret &= VM_FAULT_ERROR;
                goto out;
        }

        /* No need to invalidate - it was non-present before */
        update_mmu_cache(vma, vmf->address, vmf->pte);
unlock:
        pte_unmap_unlock(vmf->pte, vmf->ptl);
out:
        return ret;
out_nomap:
        mem_cgroup_cancel_charge(page, memcg, false);
        pte_unmap_unlock(vmf->pte, vmf->ptl);
out_page:
        unlock_page(page);
out_release:
        put_page(page);
        if (page != swapcache && swapcache) {
                unlock_page(swapcache);
                put_page(swapcache);
        }
        return ret;
}

/*
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults), and pte mapped but not yet locked.
 * We return with mmap_sem still held, but pte unmapped and unlocked.
 */
static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;
        struct mem_cgroup *memcg;
        struct page *page;
        vm_fault_t ret = 0;
        pte_t entry;

        /* File mapping without ->vm_ops ? */
        if (vma->vm_flags & VM_SHARED)
                return VM_FAULT_SIGBUS;

        /*
         * Use pte_alloc() instead of pte_alloc_map().  We can't run
         * pte_offset_map() on pmds where a huge pmd might be created
         * from a different thread.
         *
         * pte_alloc_map() is safe to use under down_write(mmap_sem) or when
         * parallel threads are excluded by other means.
         *
         * Here we only have down_read(mmap_sem).
         */
        if (pte_alloc(vma->vm_mm, vmf->pmd))
                return VM_FAULT_OOM;

        /* See the comment in pte_alloc_one_map() */
        if (unlikely(pmd_trans_unstable(vmf->pmd)))
                return 0;

        /* Use the zero-page for reads */
        if (!(vmf->flags & FAULT_FLAG_WRITE) &&
                        !mm_forbids_zeropage(vma->vm_mm)) {
                entry = pte_mkspecial(pfn_pte(my_zero_pfn(vmf->address),
                                                vma->vm_page_prot));
                vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
                                vmf->address, &vmf->ptl);
                if (!pte_none(*vmf->pte))
                        goto unlock;
                ret = check_stable_address_space(vma->vm_mm);
                if (ret)
                        goto unlock;
                /* Deliver the page fault to userland, check inside PT lock */
                if (userfaultfd_missing(vma)) {
                        pte_unmap_unlock(vmf->pte, vmf->ptl);
                        return handle_userfault(vmf, VM_UFFD_MISSING);
                }
                goto setpte;
        }

        /* Allocate our own private page. */
        if (unlikely(anon_vma_prepare(vma)))
                goto oom;
        page = alloc_zeroed_user_highpage_movable(vma, vmf->address);
        if (!page)
                goto oom;

        if (mem_cgroup_try_charge_delay(page, vma->vm_mm, GFP_KERNEL, &memcg,
                                        false))
                goto oom_free_page;

        /*
         * The memory barrier inside __SetPageUptodate makes sure that
         * preceeding stores to the page contents become visible before
         * the set_pte_at() write.
         */
        __SetPageUptodate(page);

        entry = mk_pte(page, vma->vm_page_prot);
        if (vma->vm_flags & VM_WRITE)
                entry = pte_mkwrite(pte_mkdirty(entry));

        vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
                        &vmf->ptl);
        if (!pte_none(*vmf->pte))
                goto release;

        ret = check_stable_address_space(vma->vm_mm);
        if (ret)
                goto release;

        /* Deliver the page fault to userland, check inside PT lock */
        if (userfaultfd_missing(vma)) {
                pte_unmap_unlock(vmf->pte, vmf->ptl);
                mem_cgroup_cancel_charge(page, memcg, false);
                put_page(page);
                return handle_userfault(vmf, VM_UFFD_MISSING);
        }

        inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
        page_add_new_anon_rmap(page, vma, vmf->address, false);
        mem_cgroup_commit_charge(page, memcg, false, false);
        lru_cache_add_active_or_unevictable(page, vma);
setpte:
        set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

        /* No need to invalidate - it was non-present before */
        update_mmu_cache(vma, vmf->address, vmf->pte);
unlock:
        pte_unmap_unlock(vmf->pte, vmf->ptl);
        return ret;
release:
        mem_cgroup_cancel_charge(page, memcg, false);
        put_page(page);
        goto unlock;
oom_free_page:
        put_page(page);
oom:
        return VM_FAULT_OOM;
}

/*
 * The mmap_sem must have been held on entry, and may have been
 * released depending on flags and vma->vm_ops->fault() return value.
 * See filemap_fault() and __lock_page_retry().
 */
static vm_fault_t __do_fault(struct vm_fault *vmf)
{
        struct vm_area_struct *vma = vmf->vma;
        vm_fault_t ret;

        /*
         * Preallocate pte before we take page_lock because this might lead to
         * deadlocks for memcg reclaim which waits for pages under writeback:
         *                              lock_page(A)