// SPDX-License-Identifier: GPL-2.0
/*
 * Virtual Memory Map support
 *
 * (C) 2007 sgi. Christoph Lameter.
 *
 * Virtual memory maps allow VM primitives pfn_to_page, page_to_pfn,
 * virt_to_page, page_address() to be implemented as a base offset
 * calculation without memory access.
 *
 * However, virtual mappings need a page table and TLBs. Many Linux
 * architectures already map their physical space using 1-1 mappings
 * via TLBs. For those arches the virtual memory map is essentially
 * for free if we use the same page size as the 1-1 mappings. In that
 * case the overhead consists of a few additional pages that are
 * allocated to create a view of memory for vmemmap.
 *
 * The architecture is expected to provide a vmemmap_populate() function
 * to instantiate the mapping.
 */
#include <linux/mm.h>
#include <linux/mmzone.h>
#include <linux/memblock.h>
#include <linux/memremap.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
#include <linux/sched.h>
#include <linux/pgtable.h>
#include <linux/bootmem_info.h>

#include <asm/dma.h>
#include <asm/pgalloc.h>
#include <asm/tlbflush.h>

#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
/**
 * struct vmemmap_remap_walk - walk vmemmap page table
 *
 * @remap_pte:          called for each lowest-level entry (PTE).
 * @nr_walked:          the number of walked pte.
 * @reuse_page:         the page which is reused for the tail vmemmap pages.
 * @reuse_addr:         the virtual address of the @reuse_page page.
 * @vmemmap_pages:      the list head of the vmemmap pages that can be freed
 *                      or is mapped from.
 */
struct vmemmap_remap_walk {
        void (*remap_pte)(pte_t *pte, unsigned long addr,
                          struct vmemmap_remap_walk *walk);
        unsigned long nr_walked;
        struct page *reuse_page;
        unsigned long reuse_addr;
        struct list_head *vmemmap_pages;
};

static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
{
        pmd_t __pmd;
        int i;
        unsigned long addr = start;
        struct page *page = pmd_page(*pmd);
        pte_t *pgtable = pte_alloc_one_kernel(&init_mm);

        if (!pgtable)
                return -ENOMEM;

        pmd_populate_kernel(&init_mm, &__pmd, pgtable);

        for (i = 0; i < PMD_SIZE / PAGE_SIZE; i++, addr += PAGE_SIZE) {
                pte_t entry, *pte;
                pgprot_t pgprot = PAGE_KERNEL;

                entry = mk_pte(page + i, pgprot);
                pte = pte_offset_kernel(&__pmd, addr);
                set_pte_at(&init_mm, addr, pte, entry);
        }

        spin_lock(&init_mm.page_table_lock);
        if (likely(pmd_leaf(*pmd))) {
                /*
                 * Higher order allocations from buddy allocator must be able to
                 * be treated as indepdenent small pages (as they can be freed
                 * individually).
                 */
                if (!PageReserved(page))
                        split_page(page, get_order(PMD_SIZE));

                /* Make pte visible before pmd. See comment in pmd_install(). */
                smp_wmb();
                pmd_populate_kernel(&init_mm, pmd, pgtable);
                flush_tlb_kernel_range(start, start + PMD_SIZE);
        } else {
                pte_free_kernel(&init_mm, pgtable);
        }
        spin_unlock(&init_mm.page_table_lock);

        return 0;
}

static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
{
        int leaf;

        spin_lock(&init_mm.page_table_lock);
        leaf = pmd_leaf(*pmd);
        spin_unlock(&init_mm.page_table_lock);

        if (!leaf)
                return 0;

        return __split_vmemmap_huge_pmd(pmd, start);
}

static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
                              unsigned long end,
                              struct vmemmap_remap_walk *walk)
{
        pte_t *pte = pte_offset_kernel(pmd, addr);

        /*
         * The reuse_page is found 'first' in table walk before we start
         * remapping (which is calling @walk->remap_pte).
         */
        if (!walk->reuse_page) {
                walk->reuse_page = pte_page(*pte);
                /*
                 * Because the reuse address is part of the range that we are
                 * walking, skip the reuse address range.
                 */
                addr += PAGE_SIZE;
                pte++;
                walk->nr_walked++;
        }

        for (; addr != end; addr += PAGE_SIZE, pte++) {
                walk->remap_pte(pte, addr, walk);
                walk->nr_walked++;
        }
}

static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
                             unsigned long end,
                             struct vmemmap_remap_walk *walk)
{
        pmd_t *pmd;
        unsigned long next;

        pmd = pmd_offset(pud, addr);
        do {
                int ret;

                ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
                if (ret)
                        return ret;

                next = pmd_addr_end(addr, end);
                vmemmap_pte_range(pmd, addr, next, walk);
        } while (pmd++, addr = next, addr != end);

        return 0;
}

static int vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
                             unsigned long end,
                             struct vmemmap_remap_walk *walk)
{
        pud_t *pud;
        unsigned long next;

        pud = pud_offset(p4d, addr);
        do {
                int ret;

                next = pud_addr_end(addr, end);
                ret = vmemmap_pmd_range(pud, addr, next, walk);
                if (ret)
                        return ret;
        } while (pud++, addr = next, addr != end);

        return 0;
}

static int vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
                             unsigned long end,
                             struct vmemmap_remap_walk *walk)
{
        p4d_t *p4d;
        unsigned long next;

        p4d = p4d_offset(pgd, addr);
        do {
                int ret;

                next = p4d_addr_end(addr, end);
                ret = vmemmap_pud_range(p4d, addr, next, walk);
                if (ret)
                        return ret;
        } while (p4d++, addr = next, addr != end);

        return 0;
}

static int vmemmap_remap_range(unsigned long start, unsigned long end,
                               struct vmemmap_remap_walk *walk)
{
        unsigned long addr = start;
        unsigned long next;
        pgd_t *pgd;

        VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
        VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));

        pgd = pgd_offset_k(addr);
        do {
                int ret;

                next = pgd_addr_end(addr, end);
                ret = vmemmap_p4d_range(pgd, addr, next, walk);
                if (ret)
                        return ret;
        } while (pgd++, addr = next, addr != end);

        /*
         * We only change the mapping of the vmemmap virtual address range
         * [@start + PAGE_SIZE, end), so we only need to flush the TLB which
         * belongs to the range.
         */
        flush_tlb_kernel_range(start + PAGE_SIZE, end);

        return 0;
}

/*
 * Free a vmemmap page. A vmemmap page can be allocated from the memblock
 * allocator or buddy allocator. If the PG_reserved flag is set, it means
 * that it allocated from the memblock allocator, just free it via the
 * free_bootmem_page(). Otherwise, use __free_page().
 */
static inline void free_vmemmap_page(struct page *page)
{
        if (PageReserved(page))
                free_bootmem_page(page);
        else
                __free_page(page);
}

/* Free a list of the vmemmap pages */
static void free_vmemmap_page_list(struct list_head *list)
{
        struct page *page, *next;

        list_for_each_entry_safe(page, next, list, lru) {
                list_del(&page->lru);
                free_vmemmap_page(page);
        }
}

static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
                              struct vmemmap_remap_walk *walk)
{
        /*
         * Remap the tail pages as read-only to catch illegal write operation
         * to the tail pages.
         */
        pgprot_t pgprot = PAGE_KERNEL_RO;
        pte_t entry = mk_pte(walk->reuse_page, pgprot);
        struct page *page = pte_page(*pte);

        list_add_tail(&page->lru, walk->vmemmap_pages);
        set_pte_at(&init_mm, addr, pte, entry);
}

/*
 * How many struct page structs need to be reset. When we reuse the head
 * struct page, the special metadata (e.g. page->flags or page->mapping)
 * cannot copy to the tail struct page structs. The invalid value will be
 * checked in the free_tail_pages_check(). In order to avoid the message
 * of "corrupted mapping in tail page". We need to reset at least 3 (one
 * head struct page struct and two tail struct page structs) struct page
 * structs.
 */
#define NR_RESET_STRUCT_PAGE            3

static inline void reset_struct_pages(struct page *start)
{
        int i;
        struct page *from = start + NR_RESET_STRUCT_PAGE;

        for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
                memcpy(start + i, from, sizeof(*from));
}

static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
                                struct vmemmap_remap_walk *walk)
{
        pgprot_t pgprot = PAGE_KERNEL;
        struct page *page;
        void *to;

        BUG_ON(pte_page(*pte) != walk->reuse_page);

        page = list_first_entry(walk->vmemmap_pages, struct page, lru);
        list_del(&page->lru);
        to = page_to_virt(page);
        copy_page(to, (void *)walk->reuse_addr);
        reset_struct_pages(to);

        set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
}

/**
 * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
 *                      to the page which @reuse is mapped to, then free vmemmap
 *                      which the range are mapped to.
 * @start:      start address of the vmemmap virtual address range that we want
 *              to remap.
 * @end:        end address of the vmemmap virtual address range that we want to
 *              remap.
 * @reuse:      reuse address.
 *
 * Return: %0 on success, negative error code otherwise.
 */
int vmemmap_remap_free(unsigned long start, unsigned long end,
                       unsigned long reuse)
{
        int ret;
        LIST_HEAD(vmemmap_pages);
        struct vmemmap_remap_walk walk = {
                .remap_pte      = vmemmap_remap_pte,
                .reuse_addr     = reuse,
                .vmemmap_pages  = &vmemmap_pages,
        };

        /*
         * In order to make remapping routine most efficient for the huge pages,
         * the routine of vmemmap page table walking has the following rules
         * (see more details from the vmemmap_pte_range()):
         *
         * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
         *   should be continuous.
         * - The @reuse address is part of the range [@reuse, @end) that we are
         *   walking which is passed to vmemmap_remap_range().
         * - The @reuse address is the first in the complete range.
         *
         * So we need to make sure that @start and @reuse meet the above rules.
         */
        BUG_ON(start - reuse != PAGE_SIZE);

        mmap_read_lock(&init_mm);
        ret = vmemmap_remap_range(reuse, end, &walk);
        if (ret && walk.nr_walked) {
                end = reuse + walk.nr_walked * PAGE_SIZE;
                /*
                 * vmemmap_pages contains pages from the previous
                 * vmemmap_remap_range call which failed.  These
                 * are pages which were removed from the vmemmap.
                 * They will be restored in the following call.
                 */
                walk = (struct vmemmap_remap_walk) {
                        .remap_pte      = vmemmap_restore_pte,
                        .reuse_addr     = reuse,
                        .vmemmap_pages  = &vmemmap_pages,
                };

                vmemmap_remap_range(reuse, end, &walk);
        }
        mmap_read_unlock(&init_mm);

        free_vmemmap_page_list(&vmemmap_pages);

        return ret;
}

static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
                                   gfp_t gfp_mask, struct list_head *list)
{
        unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
        int nid = page_to_nid((struct page *)start);
        struct page *page, *next;

        while (nr_pages--) {
                page = alloc_pages_node(nid, gfp_mask, 0);
                if (!page)
                        goto out;
                list_add_tail(&page->lru, list);
        }

        return 0;
out:
        list_for_each_entry_safe(page, next, list, lru)
                __free_pages(page, 0);
        return -ENOMEM;
}

/**
 * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
 *                       to the page which is from the @vmemmap_pages
 *                       respectively.
 * @start:      start address of the vmemmap virtual address range that we want
 *              to remap.
 * @end:        end address of the vmemmap virtual address range that we want to
 *              remap.
 * @reuse:      reuse address.
 * @gfp_mask:   GFP flag for allocating vmemmap pages.
 *
 * Return: %0 on success, negative error code otherwise.
 */
int vmemmap_remap_alloc(unsigned long start, unsigned long end,
                        unsigned long reuse, gfp_t gfp_mask)
{
        LIST_HEAD(vmemmap_pages);
        struct vmemmap_remap_walk walk = {
                .remap_pte      = vmemmap_restore_pte,
                .reuse_addr     = reuse,
                .vmemmap_pages  = &vmemmap_pages,
        };

        /* See the comment in the vmemmap_remap_free(). */
        BUG_ON(start - reuse != PAGE_SIZE);

        if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
                return -ENOMEM;

        mmap_read_lock(&init_mm);
        vmemmap_remap_range(reuse, end, &walk);
        mmap_read_unlock(&init_mm);

        return 0;
}
#endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */

/*
 * Allocate a block of memory to be used to back the virtual memory map
 * or to back the page tables that are used to create the mapping.
 * Uses the main allocators if they are available, else bootmem.
 */

static void * __ref __earlyonly_bootmem_alloc(int node,
                                unsigned long size,
                                unsigned long align,
                                unsigned long goal)
{
        return memblock_alloc_try_nid_raw(size, align, goal,
                                               MEMBLOCK_ALLOC_ACCESSIBLE, node);
}

void * __meminit vmemmap_alloc_block(unsigned long size, int node)
{
        /* If the main allocator is up use that, fallback to bootmem. */
        if (slab_is_available()) {
                gfp_t gfp_mask = GFP_KERNEL|__GFP_RETRY_MAYFAIL|__GFP_NOWARN;
                int order = get_order(size);
                static bool warned;
                struct page *page;

                page = alloc_pages_node(node, gfp_mask, order);
                if (page)
                        return page_address(page);

                if (!warned) {
                        warn_alloc(gfp_mask & ~__GFP_NOWARN, NULL,
                                   "vmemmap alloc failure: order:%u", order);
                        warned = true;
                }
                return NULL;
        } else
                return __earlyonly_bootmem_alloc(node, size, size,
                                __pa(MAX_DMA_ADDRESS));
}

static void * __meminit altmap_alloc_block_buf(unsigned long size,
                                               struct vmem_altmap *altmap);

/* need to make sure size is all the same during early stage */
void * __meminit vmemmap_alloc_block_buf(unsigned long size, int node,
                                         struct vmem_altmap *altmap)
{
        void *ptr;

        if (altmap)
                return altmap_alloc_block_buf(size, altmap);

        ptr = sparse_buffer_alloc(size);
        if (!ptr)
                ptr = vmemmap_alloc_block(size, node);
        return ptr;
}

static unsigned long __meminit vmem_altmap_next_pfn(struct vmem_altmap *altmap)
{
        return altmap->base_pfn + altmap->reserve + altmap->alloc
                + altmap->align;
}

static unsigned long __meminit vmem_altmap_nr_free(struct vmem_altmap *altmap)
{
        unsigned long allocated = altmap->alloc + altmap->align;

        if (altmap->free > allocated)
                return altmap->free - allocated;
        return 0;
}

static void * __meminit altmap_alloc_block_buf(unsigned long size,
                                               struct vmem_altmap *altmap)
{
        unsigned long pfn, nr_pfns, nr_align;

        if (size & ~PAGE_MASK) {
                pr_warn_once("%s: allocations must be multiple of PAGE_SIZE (%ld)\n",
                                __func__, size);
                return NULL;
        }

        pfn = vmem_altmap_next_pfn(altmap);
        nr_pfns = size >> PAGE_SHIFT;
        nr_align = 1UL << find_first_bit(&nr_pfns, BITS_PER_LONG);
        nr_align = ALIGN(pfn, nr_align) - pfn;
        if (nr_pfns + nr_align > vmem_altmap_nr_free(altmap))
                return NULL;

        altmap->alloc += nr_pfns;
        altmap->align += nr_align;
        pfn += nr_align;

        pr_debug("%s: pfn: %#lx alloc: %ld align: %ld nr: %#lx\n",
                        __func__, pfn, altmap->alloc, altmap->align, nr_pfns);
        return __va(__pfn_to_phys(pfn));
}

void __meminit vmemmap_verify(pte_t *pte, int node,
                                unsigned long start, unsigned long end)
{
        unsigned long pfn = pte_pfn(*pte);
        int actual_node = early_pfn_to_nid(pfn);

        if (node_distance(actual_node, node) > LOCAL_DISTANCE)
                pr_warn("[%lx-%lx] potential offnode page_structs\n",
                        start, end - 1);
}

pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
                                       struct vmem_altmap *altmap,
                                       struct page *reuse)
{
        pte_t *pte = pte_offset_kernel(pmd, addr);
        if (pte_none(*pte)) {
                pte_t entry;
                void *p;

                if (!reuse) {
                        p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
                        if (!p)
                                return NULL;
                } else {
                        /*
                         * When a PTE/PMD entry is freed from the init_mm
                         * there's a a free_pages() call to this page allocated
                         * above. Thus this get_page() is paired with the
                         * put_page_testzero() on the freeing path.
                         * This can only called by certain ZONE_DEVICE path,
                         * and through vmemmap_populate_compound_pages() when
                         * slab is available.
                         */
                        get_page(reuse);
                        p = page_to_virt(reuse);
                }
                entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL);
                set_pte_at(&init_mm, addr, pte, entry);
        }
        return pte;
}

static void * __meminit vmemmap_alloc_block_zero(unsigned long size, int node)
{
        void *p = vmemmap_alloc_block(size, node);

        if (!p)
                return NULL;
        memset(p, 0, size);

        return p;
}

pmd_t * __meminit vmemmap_pmd_populate(pud_t *pud, unsigned long addr, int node)
{
        pmd_t *pmd = pmd_offset(pud, addr);
        if (pmd_none(*pmd)) {
                void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
                if (!p)
                        return NULL;
                pmd_populate_kernel(&init_mm, pmd, p);
        }
        return pmd;
}

pud_t * __meminit vmemmap_pud_populate(p4d_t *p4d, unsigned long addr, int node)
{
        pud_t *pud = pud_offset(p4d, addr);
        if (pud_none(*pud)) {
                void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
                if (!p)
                        return NULL;
                pud_populate(&init_mm, pud, p);
        }
        return pud;
}

p4d_t * __meminit vmemmap_p4d_populate(pgd_t *pgd, unsigned long addr, int node)
{
        p4d_t *p4d = p4d_offset(pgd, addr);
        if (p4d_none(*p4d)) {
                void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
                if (!p)
                        return NULL;
                p4d_populate(&init_mm, p4d, p);
        }
        return p4d;
}

pgd_t * __meminit vmemmap_pgd_populate(unsigned long addr, int node)
{
        pgd_t *pgd = pgd_offset_k(addr);
        if (pgd_none(*pgd)) {
                void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
                if (!p)
                        return NULL;
                pgd_populate(&init_mm, pgd, p);
        }
        return pgd;
}

static pte_t * __meminit vmemmap_populate_address(unsigned long addr, int node,
                                              struct vmem_altmap *altmap,
                                              struct page *reuse)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;

        pgd = vmemmap_pgd_populate(addr, node);
        if (!pgd)
                return NULL;
        p4d = vmemmap_p4d_populate(pgd, addr, node);
        if (!p4d)
                return NULL;
        pud = vmemmap_pud_populate(p4d, addr, node);
        if (!pud)
                return NULL;
        pmd = vmemmap_pmd_populate(pud, addr, node);
        if (!pmd)
                return NULL;
        pte = vmemmap_pte_populate(pmd, addr, node, altmap, reuse);
        if (!pte)
                return NULL;
        vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);

        return pte;
}

static int __meminit vmemmap_populate_range(unsigned long start,
                                            unsigned long end, int node,
                                            struct vmem_altmap *altmap,
                                            struct page *reuse)
{
        unsigned long addr = start;
        pte_t *pte;

        for (; addr < end; addr += PAGE_SIZE) {
                pte = vmemmap_populate_address(addr, node, altmap, reuse);
                if (!pte)
                        return -ENOMEM;
        }

        return 0;
}

int __meminit vmemmap_populate_basepages(unsigned long start, unsigned long end,
                                         int node, struct vmem_altmap *altmap)
{
        return vmemmap_populate_range(start, end, node, altmap, NULL);
}

/*
 * For compound pages bigger than section size (e.g. x86 1G compound
 * pages with 2M subsection size) fill the rest of sections as tail
 * pages.
 *
 * Note that memremap_pages() resets @nr_range value and will increment
 * it after each range successful onlining. Thus the value or @nr_range
 * at section memmap populate corresponds to the in-progress range
 * being onlined here.
 */
static bool __meminit reuse_compound_section(unsigned long start_pfn,
                                             struct dev_pagemap *pgmap)
{
        unsigned long nr_pages = pgmap_vmemmap_nr(pgmap);
        unsigned long offset = start_pfn -
                PHYS_PFN(pgmap->ranges[pgmap->nr_range].start);

        return !IS_ALIGNED(offset, nr_pages) && nr_pages > PAGES_PER_SUBSECTION;
}

static pte_t * __meminit compound_section_tail_page(unsigned long addr)
{
        pte_t *pte;

        addr -= PAGE_SIZE;

        /*
         * Assuming sections are populated sequentially, the previous section's
         * page data can be reused.
         */
        pte = pte_offset_kernel(pmd_off_k(addr), addr);
        if (!pte)
                return NULL;

        return pte;
}

static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
                                                     unsigned long start,
                                                     unsigned long end, int node,
                                                     struct dev_pagemap *pgmap)
{
        unsigned long size, addr;
        pte_t *pte;
        int rc;

        if (reuse_compound_section(start_pfn, pgmap)) {
                pte = compound_section_tail_page(start);
                if (!pte)
                        return -ENOMEM;

                /*
                 * Reuse the page that was populated in the prior iteration
                 * with just tail struct pages.
                 */
                return vmemmap_populate_range(start, end, node, NULL,
                                              pte_page(*pte));
        }

        size = min(end - start, pgmap_vmemmap_nr(pgmap) * sizeof(struct page));
        for (addr = start; addr < end; addr += size) {
                unsigned long next = addr, last = addr + size;

                /* Populate the head page vmemmap page */
                pte = vmemmap_populate_address(addr, node, NULL, NULL);
                if (!pte)
                        return -ENOMEM;

                /* Populate the tail pages vmemmap page */
                next = addr + PAGE_SIZE;
                pte = vmemmap_populate_address(next, node, NULL, NULL);
                if (!pte)
                        return -ENOMEM;

                /*
                 * Reuse the previous page for the rest of tail pages
                 * See layout diagram in Documentation/vm/vmemmap_dedup.rst
                 */
                next += PAGE_SIZE;
                rc = vmemmap_populate_range(next, last, node, NULL,
                                            pte_page(*pte));
                if (rc)
                        return -ENOMEM;
        }

        return 0;
}

struct page * __meminit __populate_section_memmap(unsigned long pfn,
                unsigned long nr_pages, int nid, struct vmem_altmap *altmap,
                struct dev_pagemap *pgmap)
{
        unsigned long start = (unsigned long) pfn_to_page(pfn);
        unsigned long end = start + nr_pages * sizeof(struct page);
        int r;

        if (WARN_ON_ONCE(!IS_ALIGNED(pfn, PAGES_PER_SUBSECTION) ||
                !IS_ALIGNED(nr_pages, PAGES_PER_SUBSECTION)))
                return NULL;

        if (is_power_of_2(sizeof(struct page)) &&
            pgmap && pgmap_vmemmap_nr(pgmap) > 1 && !altmap)
                r = vmemmap_populate_compound_pages(pfn, start, end, nid, pgmap);
        else
                r = vmemmap_populate(start, end, nid, altmap);

        if (r < 0)
                return NULL;

        return pfn_to_page(pfn);
}