/*
 * Lockless get_user_pages_fast for x86
 *
 * Copyright (C) 2008 Nick Piggin
 * Copyright (C) 2008 Novell Inc.
 */
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/vmstat.h>
#include <linux/highmem.h>
#include <linux/swap.h>
#include <linux/memremap.h>

#include <asm/pgtable.h>

static inline pte_t gup_get_pte(pte_t *ptep)
{
#ifndef CONFIG_X86_PAE
        return ACCESS_ONCE(*ptep);
#else
        /*
         * With get_user_pages_fast, we walk down the pagetables without taking
         * any locks.  For this we would like to load the pointers atomically,
         * but that is not possible (without expensive cmpxchg8b) on PAE.  What
         * we do have is the guarantee that a pte will only either go from not
         * present to present, or present to not present or both -- it will not
         * switch to a completely different present page without a TLB flush in
         * between; something that we are blocking by holding interrupts off.
         *
         * Setting ptes from not present to present goes:
         * ptep->pte_high = h;
         * smp_wmb();
         * ptep->pte_low = l;
         *
         * And present to not present goes:
         * ptep->pte_low = 0;
         * smp_wmb();
         * ptep->pte_high = 0;
         *
         * We must ensure here that the load of pte_low sees l iff pte_high
         * sees h. We load pte_high *after* loading pte_low, which ensures we
         * don't see an older value of pte_high.  *Then* we recheck pte_low,
         * which ensures that we haven't picked up a changed pte high. We might
         * have got rubbish values from pte_low and pte_high, but we are
         * guaranteed that pte_low will not have the present bit set *unless*
         * it is 'l'. And get_user_pages_fast only operates on present ptes, so
         * we're safe.
         *
         * gup_get_pte should not be used or copied outside gup.c without being
         * very careful -- it does not atomically load the pte or anything that
         * is likely to be useful for you.
         */
        pte_t pte;

retry:
        pte.pte_low = ptep->pte_low;
        smp_rmb();
        pte.pte_high = ptep->pte_high;
        smp_rmb();
        if (unlikely(pte.pte_low != ptep->pte_low))
                goto retry;

        return pte;
#endif
}

static void undo_dev_pagemap(int *nr, int nr_start, struct page **pages)
{
        while ((*nr) - nr_start) {
                struct page *page = pages[--(*nr)];

                ClearPageReferenced(page);
                put_page(page);
        }
}

/*
 * 'pteval' can come from a pte, pmd or pud.  We only check
 * _PAGE_PRESENT, _PAGE_USER, and _PAGE_RW in here which are the
 * same value on all 3 types.
 */
static inline int pte_allows_gup(unsigned long pteval, int write)
{
        unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;

        if (write)
                need_pte_bits |= _PAGE_RW;

        if ((pteval & need_pte_bits) != need_pte_bits)
                return 0;

        return 1;
}

/*
 * The performance critical leaf functions are made noinline otherwise gcc
 * inlines everything into a single function which results in too much
 * register pressure.
 */
static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
{
        struct dev_pagemap *pgmap = NULL;
        int nr_start = *nr;
        pte_t *ptep;

        ptep = pte_offset_map(&pmd, addr);
        do {
                pte_t pte = gup_get_pte(ptep);
                struct page *page;

                /* Similar to the PMD case, NUMA hinting must take slow path */
                if (pte_numa(pte)) {
                        pte_unmap(ptep);
                        return 0;
                }

                if (!pte_allows_gup(pte_val(pte), write)) {
                        pte_unmap(ptep);
                        return 0;
                }

                if (pte_devmap(pte)) {
                        pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
                        if (unlikely(!pgmap)) {
                                undo_dev_pagemap(nr, nr_start, pages);
                                pte_unmap(ptep);
                                return 0;
                        }
                } else if (pte_special(pte)) {
                        pte_unmap(ptep);
                        return 0;
                }
                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                page = pte_page(pte);
                get_page(page);
                put_dev_pagemap(pgmap);
                SetPageReferenced(page);
                pages[*nr] = page;
                (*nr)++;

        } while (ptep++, addr += PAGE_SIZE, addr != end);
        pte_unmap(ptep - 1);

        return 1;
}

static inline void get_head_page_multiple(struct page *page, int nr)
{
        VM_BUG_ON_PAGE(page != compound_head(page), page);
        VM_BUG_ON_PAGE(page_count(page) == 0, page);
        page_ref_add(page, nr);
        SetPageReferenced(page);
}

static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
{
        pte_t pte = *(pte_t *)&pmd;
        struct page *head, *page;
        int refs;

        if (!pte_allows_gup(pte_val(pte), write))
                return 0;
        /* hugepages are never "special" */
        VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

        refs = 0;
        head = pte_page(pte);
        page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
        do {
                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                if (PageTail(page))
                        get_huge_page_tail(page);
                (*nr)++;
                page++;
                refs++;
        } while (addr += PAGE_SIZE, addr != end);
        get_head_page_multiple(head, refs);

        return 1;
}

static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
                int write, struct page **pages, int *nr)
{
        unsigned long next;
        pmd_t *pmdp;

        pmdp = pmd_offset(&pud, addr);
        do {
                pmd_t pmd = *pmdp;

                next = pmd_addr_end(addr, end);
                /*
                 * The pmd_trans_splitting() check below explains why
                 * pmdp_splitting_flush has to flush the tlb, to stop
                 * this gup-fast code from running while we set the
                 * splitting bit in the pmd. Returning zero will take
                 * the slow path that will call wait_split_huge_page()
                 * if the pmd is still in splitting state. gup-fast
                 * can't because it has irq disabled and
                 * wait_split_huge_page() would never return as the
                 * tlb flush IPI wouldn't run.
                 */
                if (pmd_none(pmd) || pmd_trans_splitting(pmd))
                        return 0;
                if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
                        /*
                         * NUMA hinting faults need to be handled in the GUP
                         * slowpath for accounting purposes and so that they
                         * can be serialised against THP migration.
                         */
                        if (pmd_numa(pmd))
                                return 0;
                        if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
                                return 0;
                } else {
                        if (!gup_pte_range(pmd, addr, next, write, pages, nr))
                                return 0;
                }
        } while (pmdp++, addr = next, addr != end);

        return 1;
}

static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
{
        pte_t pte = *(pte_t *)&pud;
        struct page *head, *page;
        int refs;

        if (!pte_allows_gup(pte_val(pte), write))
                return 0;
        /* hugepages are never "special" */
        VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

        refs = 0;
        head = pte_page(pte);
        page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
        do {
                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                if (PageTail(page))
                        get_huge_page_tail(page);
                (*nr)++;
                page++;
                refs++;
        } while (addr += PAGE_SIZE, addr != end);
        get_head_page_multiple(head, refs);

        return 1;
}

static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
                        int write, struct page **pages, int *nr)
{
        unsigned long next;
        pud_t *pudp;

        pudp = pud_offset(&pgd, addr);
        do {
                pud_t pud = *pudp;

                next = pud_addr_end(addr, end);
                if (pud_none(pud))
                        return 0;
                if (unlikely(pud_large(pud))) {
                        if (!gup_huge_pud(pud, addr, next, write, pages, nr))
                                return 0;
                } else {
                        if (!gup_pmd_range(pud, addr, next, write, pages, nr))
                                return 0;
                }
        } while (pudp++, addr = next, addr != end);

        return 1;
}

/*
 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 * back to the regular GUP.
 */
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
                          struct page **pages)
{
        struct mm_struct *mm = current->mm;
        unsigned long addr, len, end;
        unsigned long next;
        unsigned long flags;
        pgd_t *pgdp;
        int nr = 0;

        start &= PAGE_MASK;
        addr = start;
        len = (unsigned long) nr_pages << PAGE_SHIFT;
        end = start + len;
        if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
                                        (void __user *)start, len)))
                return 0;

        /*
         * XXX: batch / limit 'nr', to avoid large irq off latency
         * needs some instrumenting to determine the common sizes used by
         * important workloads (eg. DB2), and whether limiting the batch size
         * will decrease performance.
         *
         * It seems like we're in the clear for the moment. Direct-IO is
         * the main guy that batches up lots of get_user_pages, and even
         * they are limited to 64-at-a-time which is not so many.
         */
        /*
         * This doesn't prevent pagetable teardown, but does prevent
         * the pagetables and pages from being freed on x86.
         *
         * So long as we atomically load page table pointers versus teardown
         * (which we do on x86, with the above PAE exception), we can follow the
         * address down to the the page and take a ref on it.
         */
        local_irq_save(flags);
        pgdp = pgd_offset(mm, addr);
        do {
                pgd_t pgd = *pgdp;

                next = pgd_addr_end(addr, end);
                if (pgd_none(pgd))
                        break;
                if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
                        break;
        } while (pgdp++, addr = next, addr != end);
        local_irq_restore(flags);

        return nr;
}

/**
 * get_user_pages_fast() - pin user pages in memory
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @write:      whether pages will be written to
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 *
 * Attempt to pin user pages in memory without taking mm->mmap_sem.
 * If not successful, it will fall back to taking the lock and
 * calling get_user_pages().
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno.
 */
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
                        struct page **pages)
{
        struct mm_struct *mm = current->mm;
        unsigned long addr, len, end;
        unsigned long next;
        pgd_t *pgdp;
        int nr = 0;

        start &= PAGE_MASK;
        addr = start;
        len = (unsigned long) nr_pages << PAGE_SHIFT;

        end = start + len;
        if (end < start)
                goto slow_irqon;

#ifdef CONFIG_X86_64
        if (end >> __VIRTUAL_MASK_SHIFT)
                goto slow_irqon;
#endif

        /*
         * XXX: batch / limit 'nr', to avoid large irq off latency
         * needs some instrumenting to determine the common sizes used by
         * important workloads (eg. DB2), and whether limiting the batch size
         * will decrease performance.
         *
         * It seems like we're in the clear for the moment. Direct-IO is
         * the main guy that batches up lots of get_user_pages, and even
         * they are limited to 64-at-a-time which is not so many.
         */
        /*
         * This doesn't prevent pagetable teardown, but does prevent
         * the pagetables and pages from being freed on x86.
         *
         * So long as we atomically load page table pointers versus teardown
         * (which we do on x86, with the above PAE exception), we can follow the
         * address down to the the page and take a ref on it.
         */
        local_irq_disable();
        pgdp = pgd_offset(mm, addr);
        do {
                pgd_t pgd = *pgdp;

                next = pgd_addr_end(addr, end);
                if (pgd_none(pgd))
                        goto slow;
                if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
                        goto slow;
        } while (pgdp++, addr = next, addr != end);
        local_irq_enable();

        VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
        return nr;

        {
                int ret;

slow:
                local_irq_enable();
slow_irqon:
                /* Try to get the remaining pages with get_user_pages */
                start += nr << PAGE_SHIFT;
                pages += nr;

                ret = get_user_pages_unlocked(current, mm, start,
                                              (end - start) >> PAGE_SHIFT,
                                              write, 0, pages);

                /* Have to be a bit careful with return values */
                if (nr > 0) {
                        if (ret < 0)
                                ret = nr;
                        else
                                ret += nr;
                }

                return ret;
        }
}