Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost

[linux-stable.git] / mm / hugetlb_vmemmap.c

// SPDX-License-Identifier: GPL-2.0
/*
 * HugeTLB Vmemmap Optimization (HVO)
 *
 * Copyright (c) 2020, ByteDance. All rights reserved.
 *
 *     Author: Muchun Song <songmuchun@bytedance.com>
 *
 * See Documentation/mm/vmemmap_dedup.rst
 */
#define pr_fmt(fmt)     "HugeTLB: " fmt

#include <linux/pgtable.h>
#include <linux/moduleparam.h>
#include <linux/bootmem_info.h>
#include <linux/mmdebug.h>
#include <linux/pagewalk.h>
#include <asm/pgalloc.h>
#include <asm/tlbflush.h>
#include "hugetlb_vmemmap.h"

/**
 * 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.
 * @flags:              used to modify behavior in vmemmap page table walking
 *                      operations.
 */
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;

/* Skip the TLB flush when we split the PMD */
#define VMEMMAP_SPLIT_NO_TLB_FLUSH      BIT(0)
/* Skip the TLB flush when we remap the PTE */
#define VMEMMAP_REMAP_NO_TLB_FLUSH      BIT(1)
/* synchronize_rcu() to avoid writes from page_ref_add_unless() */
#define VMEMMAP_SYNCHRONIZE_RCU         BIT(2)
        unsigned long           flags;
};

static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start,
                             struct vmemmap_remap_walk *walk)
{
        pmd_t __pmd;
        int i;
        unsigned long addr = start;
        pte_t *pgtable;

        pgtable = pte_alloc_one_kernel(&init_mm);
        if (!pgtable)
                return -ENOMEM;

        pmd_populate_kernel(&init_mm, &__pmd, pgtable);

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

                entry = mk_pte(head + 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(head))
                        split_page(head, get_order(PMD_SIZE));

                /* Make pte visible before pmd. See comment in pmd_install(). */
                smp_wmb();
                pmd_populate_kernel(&init_mm, pmd, pgtable);
                if (!(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH))
                        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 vmemmap_pmd_entry(pmd_t *pmd, unsigned long addr,
                             unsigned long next, struct mm_walk *walk)
{
        int ret = 0;
        struct page *head;
        struct vmemmap_remap_walk *vmemmap_walk = walk->private;

        /* Only splitting, not remapping the vmemmap pages. */
        if (!vmemmap_walk->remap_pte)
                walk->action = ACTION_CONTINUE;

        spin_lock(&init_mm.page_table_lock);
        head = pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL;
        /*
         * Due to HugeTLB alignment requirements and the vmemmap
         * pages being at the start of the hotplugged memory
         * region in memory_hotplug.memmap_on_memory case. Checking
         * the vmemmap page associated with the first vmemmap page
         * if it is self-hosted is sufficient.
         *
         * [                  hotplugged memory                  ]
         * [        section        ][...][        section        ]
         * [ vmemmap ][              usable memory               ]
         *   ^  | ^                        |
         *   +--+ |                        |
         *        +------------------------+
         */
        if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG) && unlikely(!vmemmap_walk->nr_walked)) {
                struct page *page = head ? head + pte_index(addr) :
                                    pte_page(ptep_get(pte_offset_kernel(pmd, addr)));

                if (PageVmemmapSelfHosted(page))
                        ret = -ENOTSUPP;
        }
        spin_unlock(&init_mm.page_table_lock);
        if (!head || ret)
                return ret;

        return vmemmap_split_pmd(pmd, head, addr & PMD_MASK, vmemmap_walk);
}

static int vmemmap_pte_entry(pte_t *pte, unsigned long addr,
                             unsigned long next, struct mm_walk *walk)
{
        struct vmemmap_remap_walk *vmemmap_walk = walk->private;

        /*
         * The reuse_page is found 'first' in page table walking before
         * starting remapping.
         */
        if (!vmemmap_walk->reuse_page)
                vmemmap_walk->reuse_page = pte_page(ptep_get(pte));
        else
                vmemmap_walk->remap_pte(pte, addr, vmemmap_walk);
        vmemmap_walk->nr_walked++;

        return 0;
}

static const struct mm_walk_ops vmemmap_remap_ops = {
        .pmd_entry      = vmemmap_pmd_entry,
        .pte_entry      = vmemmap_pte_entry,
};

static int vmemmap_remap_range(unsigned long start, unsigned long end,
                               struct vmemmap_remap_walk *walk)
{
        int ret;

        VM_BUG_ON(!PAGE_ALIGNED(start | end));

        mmap_read_lock(&init_mm);
        ret = walk_page_range_novma(&init_mm, start, end, &vmemmap_remap_ops,
                                    NULL, walk);
        mmap_read_unlock(&init_mm);
        if (ret)
                return ret;

        if (walk->remap_pte && !(walk->flags & VMEMMAP_REMAP_NO_TLB_FLUSH))
                flush_tlb_kernel_range(start, 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)) {
                memmap_boot_pages_add(-1);
                free_bootmem_page(page);
        } else {
                memmap_pages_add(-1);
                __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)
                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;
        struct page *page = pte_page(ptep_get(pte));
        pte_t entry;

        /* Remapping the head page requires r/w */
        if (unlikely(addr == walk->reuse_addr)) {
                pgprot = PAGE_KERNEL;
                list_del(&walk->reuse_page->lru);

                /*
                 * Makes sure that preceding stores to the page contents from
                 * vmemmap_remap_free() become visible before the set_pte_at()
                 * write.
                 */
                smp_wmb();
        }

        entry = mk_pte(walk->reuse_page, pgprot);
        list_add(&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_page_prepare(). 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)
{
        struct page *from = start + NR_RESET_STRUCT_PAGE;

        BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
        memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE);
}

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(ptep_get(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);

        /*
         * Makes sure that preceding stores to the page contents become visible
         * before the set_pte_at() write.
         */
        smp_wmb();
        set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
}

/**
 * vmemmap_remap_split - split the vmemmap virtual address range [@start, @end)
 *                      backing PMDs of the directmap into PTEs
 * @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.
 */
static int vmemmap_remap_split(unsigned long start, unsigned long end,
                               unsigned long reuse)
{
        struct vmemmap_remap_walk walk = {
                .remap_pte      = NULL,
                .flags          = VMEMMAP_SPLIT_NO_TLB_FLUSH,
        };

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

        return vmemmap_remap_range(reuse, end, &walk);
}

/**
 * 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.
 * @vmemmap_pages: list to deposit vmemmap pages to be freed.  It is callers
 *              responsibility to free pages.
 * @flags:      modifications to vmemmap_remap_walk flags
 *
 * Return: %0 on success, negative error code otherwise.
 */
static int vmemmap_remap_free(unsigned long start, unsigned long end,
                              unsigned long reuse,
                              struct list_head *vmemmap_pages,
                              unsigned long flags)
{
        int ret;
        struct vmemmap_remap_walk walk = {
                .remap_pte      = vmemmap_remap_pte,
                .reuse_addr     = reuse,
                .vmemmap_pages  = vmemmap_pages,
                .flags          = flags,
        };
        int nid = page_to_nid((struct page *)reuse);
        gfp_t gfp_mask = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;

        /*
         * Allocate a new head vmemmap page to avoid breaking a contiguous
         * block of struct page memory when freeing it back to page allocator
         * in free_vmemmap_page_list(). This will allow the likely contiguous
         * struct page backing memory to be kept contiguous and allowing for
         * more allocations of hugepages. Fallback to the currently
         * mapped head page in case should it fail to allocate.
         */
        walk.reuse_page = alloc_pages_node(nid, gfp_mask, 0);
        if (walk.reuse_page) {
                copy_page(page_to_virt(walk.reuse_page),
                          (void *)walk.reuse_addr);
                list_add(&walk.reuse_page->lru, vmemmap_pages);
                memmap_pages_add(1);
        }

        /*
         * 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);

        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,
                        .flags          = 0,
                };

                vmemmap_remap_range(reuse, end, &walk);
        }

        return ret;
}

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

        for (i = 0; i < nr_pages; i++) {
                page = alloc_pages_node(nid, gfp_mask, 0);
                if (!page)
                        goto out;
                list_add(&page->lru, list);
        }
        memmap_pages_add(nr_pages);

        return 0;
out:
        list_for_each_entry_safe(page, next, list, lru)
                __free_page(page);
        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.
 * @flags:      modifications to vmemmap_remap_walk flags
 *
 * Return: %0 on success, negative error code otherwise.
 */
static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
                               unsigned long reuse, unsigned long flags)
{
        LIST_HEAD(vmemmap_pages);
        struct vmemmap_remap_walk walk = {
                .remap_pte      = vmemmap_restore_pte,
                .reuse_addr     = reuse,
                .vmemmap_pages  = &vmemmap_pages,
                .flags          = flags,
        };

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

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

        return vmemmap_remap_range(reuse, end, &walk);
}

DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);

static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
core_param(hugetlb_free_vmemmap, vmemmap_optimize_enabled, bool, 0);

static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
                                           struct folio *folio, unsigned long flags)
{
        int ret;
        unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
        unsigned long vmemmap_reuse;

        VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
        VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);

        if (!folio_test_hugetlb_vmemmap_optimized(folio))
                return 0;

        if (flags & VMEMMAP_SYNCHRONIZE_RCU)
                synchronize_rcu();

        vmemmap_end     = vmemmap_start + hugetlb_vmemmap_size(h);
        vmemmap_reuse   = vmemmap_start;
        vmemmap_start   += HUGETLB_VMEMMAP_RESERVE_SIZE;

        /*
         * The pages which the vmemmap virtual address range [@vmemmap_start,
         * @vmemmap_end) are mapped to are freed to the buddy allocator, and
         * the range is mapped to the page which @vmemmap_reuse is mapped to.
         * When a HugeTLB page is freed to the buddy allocator, previously
         * discarded vmemmap pages must be allocated and remapping.
         */
        ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse, flags);
        if (!ret) {
                folio_clear_hugetlb_vmemmap_optimized(folio);
                static_branch_dec(&hugetlb_optimize_vmemmap_key);
        }

        return ret;
}

/**
 * hugetlb_vmemmap_restore_folio - restore previously optimized (by
 *                              hugetlb_vmemmap_optimize_folio()) vmemmap pages which
 *                              will be reallocated and remapped.
 * @h:          struct hstate.
 * @folio:     the folio whose vmemmap pages will be restored.
 *
 * Return: %0 if @folio's vmemmap pages have been reallocated and remapped,
 * negative error code otherwise.
 */
int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
{
        return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
}

/**
 * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
 * @h:                  hstate.
 * @folio_list:         list of folios.
 * @non_hvo_folios:     Output list of folios for which vmemmap exists.
 *
 * Return: number of folios for which vmemmap was restored, or an error code
 *              if an error was encountered restoring vmemmap for a folio.
 *              Folios that have vmemmap are moved to the non_hvo_folios
 *              list.  Processing of entries stops when the first error is
 *              encountered. The folio that experienced the error and all
 *              non-processed folios will remain on folio_list.
 */
long hugetlb_vmemmap_restore_folios(const struct hstate *h,
                                        struct list_head *folio_list,
                                        struct list_head *non_hvo_folios)
{
        struct folio *folio, *t_folio;
        long restored = 0;
        long ret = 0;
        unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;

        list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
                if (folio_test_hugetlb_vmemmap_optimized(folio)) {
                        ret = __hugetlb_vmemmap_restore_folio(h, folio, flags);
                        /* only need to synchronize_rcu() once for each batch */
                        flags &= ~VMEMMAP_SYNCHRONIZE_RCU;

                        if (ret)
                                break;
                        restored++;
                }

                /* Add non-optimized folios to output list */
                list_move(&folio->lru, non_hvo_folios);
        }

        if (restored)
                flush_tlb_all();
        if (!ret)
                ret = restored;
        return ret;
}

/* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
static bool vmemmap_should_optimize_folio(const struct hstate *h, struct folio *folio)
{
        if (folio_test_hugetlb_vmemmap_optimized(folio))
                return false;

        if (!READ_ONCE(vmemmap_optimize_enabled))
                return false;

        if (!hugetlb_vmemmap_optimizable(h))
                return false;

        return true;
}

static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
                                            struct folio *folio,
                                            struct list_head *vmemmap_pages,
                                            unsigned long flags)
{
        int ret = 0;
        unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
        unsigned long vmemmap_reuse;

        VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
        VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);

        if (!vmemmap_should_optimize_folio(h, folio))
                return ret;

        static_branch_inc(&hugetlb_optimize_vmemmap_key);

        if (flags & VMEMMAP_SYNCHRONIZE_RCU)
                synchronize_rcu();
        /*
         * Very Subtle
         * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
         * immediately after remapping.  As a result, subsequent accesses
         * and modifications to struct pages associated with the hugetlb
         * page could be to the OLD struct pages.  Set the vmemmap optimized
         * flag here so that it is copied to the new head page.  This keeps
         * the old and new struct pages in sync.
         * If there is an error during optimization, we will immediately FLUSH
         * the TLB and clear the flag below.
         */
        folio_set_hugetlb_vmemmap_optimized(folio);

        vmemmap_end     = vmemmap_start + hugetlb_vmemmap_size(h);
        vmemmap_reuse   = vmemmap_start;
        vmemmap_start   += HUGETLB_VMEMMAP_RESERVE_SIZE;

        /*
         * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
         * to the page which @vmemmap_reuse is mapped to.  Add pages previously
         * mapping the range to vmemmap_pages list so that they can be freed by
         * the caller.
         */
        ret = vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse,
                                 vmemmap_pages, flags);
        if (ret) {
                static_branch_dec(&hugetlb_optimize_vmemmap_key);
                folio_clear_hugetlb_vmemmap_optimized(folio);
        }

        return ret;
}

/**
 * hugetlb_vmemmap_optimize_folio - optimize @folio's vmemmap pages.
 * @h:          struct hstate.
 * @folio:     the folio whose vmemmap pages will be optimized.
 *
 * This function only tries to optimize @folio's vmemmap pages and does not
 * guarantee that the optimization will succeed after it returns. The caller
 * can use folio_test_hugetlb_vmemmap_optimized(@folio) to detect if @folio's
 * vmemmap pages have been optimized.
 */
void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
{
        LIST_HEAD(vmemmap_pages);

        __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
        free_vmemmap_page_list(&vmemmap_pages);
}

static int hugetlb_vmemmap_split_folio(const struct hstate *h, struct folio *folio)
{
        unsigned long vmemmap_start = (unsigned long)&folio->page, vmemmap_end;
        unsigned long vmemmap_reuse;

        if (!vmemmap_should_optimize_folio(h, folio))
                return 0;

        vmemmap_end     = vmemmap_start + hugetlb_vmemmap_size(h);
        vmemmap_reuse   = vmemmap_start;
        vmemmap_start   += HUGETLB_VMEMMAP_RESERVE_SIZE;

        /*
         * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
         * @vmemmap_end]
         */
        return vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
}

void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
{
        struct folio *folio;
        LIST_HEAD(vmemmap_pages);
        unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;

        list_for_each_entry(folio, folio_list, lru) {
                int ret = hugetlb_vmemmap_split_folio(h, folio);

                /*
                 * Spliting the PMD requires allocating a page, thus lets fail
                 * early once we encounter the first OOM. No point in retrying
                 * as it can be dynamically done on remap with the memory
                 * we get back from the vmemmap deduplication.
                 */
                if (ret == -ENOMEM)
                        break;
        }

        flush_tlb_all();

        list_for_each_entry(folio, folio_list, lru) {
                int ret;

                ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
                /* only need to synchronize_rcu() once for each batch */
                flags &= ~VMEMMAP_SYNCHRONIZE_RCU;

                /*
                 * Pages to be freed may have been accumulated.  If we
                 * encounter an ENOMEM,  free what we have and try again.
                 * This can occur in the case that both spliting fails
                 * halfway and head page allocation also failed. In this
                 * case __hugetlb_vmemmap_optimize_folio() would free memory
                 * allowing more vmemmap remaps to occur.
                 */
                if (ret == -ENOMEM && !list_empty(&vmemmap_pages)) {
                        flush_tlb_all();
                        free_vmemmap_page_list(&vmemmap_pages);
                        INIT_LIST_HEAD(&vmemmap_pages);
                        __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
                }
        }

        flush_tlb_all();
        free_vmemmap_page_list(&vmemmap_pages);
}

static struct ctl_table hugetlb_vmemmap_sysctls[] = {
        {
                .procname       = "hugetlb_optimize_vmemmap",
                .data           = &vmemmap_optimize_enabled,
                .maxlen         = sizeof(vmemmap_optimize_enabled),
                .mode           = 0644,
                .proc_handler   = proc_dobool,
        },
};

static int __init hugetlb_vmemmap_init(void)
{
        const struct hstate *h;

        /* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
        BUILD_BUG_ON(__NR_USED_SUBPAGE > HUGETLB_VMEMMAP_RESERVE_PAGES);

        for_each_hstate(h) {
                if (hugetlb_vmemmap_optimizable(h)) {
                        register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
                        break;
                }
        }
        return 0;
}
late_initcall(hugetlb_vmemmap_init);