// SPDX-License-Identifier: GPL-2.0 /* * HugeTLB Vmemmap Optimization (HVO) * * Copyright (c) 2020, ByteDance. All rights reserved. * * Author: Muchun Song * * See Documentation/mm/vmemmap_dedup.rst */ #define pr_fmt(fmt) "HugeTLB: " fmt #include #include #include #include #include #include #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) unsigned long flags; }; static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start, bool flush) { pmd_t __pmd; int i; unsigned long addr = start; struct page *head; pte_t *pgtable; spin_lock(&init_mm.page_table_lock); head = pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL; spin_unlock(&init_mm.page_table_lock); if (!head) return 0; 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 (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 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(ptep_get(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, !(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH)); if (ret) return ret; next = pmd_addr_end(addr, end); /* * We are only splitting, not remapping the hugetlb vmemmap * pages. */ if (!walk->remap_pte) continue; 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(!PAGE_ALIGNED(start)); VM_BUG_ON(!PAGE_ALIGNED(end)); 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); 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)) 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) 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) { int ret; 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); mmap_read_lock(&init_mm); ret = vmemmap_remap_range(reuse, end, &walk); mmap_read_unlock(&init_mm); return ret; } /** * 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); } /* * 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, .flags = 0, }; vmemmap_remap_range(reuse, end, &walk); } mmap_read_unlock(&init_mm); 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; while (nr_pages--) { page = alloc_pages_node(nid, gfp_mask, 0); if (!page) goto out; list_add(&page->lru, list); } 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; mmap_read_lock(&init_mm); vmemmap_remap_range(reuse, end, &walk); mmap_read_unlock(&init_mm); return 0; } 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; struct page *head = &folio->page; unsigned long vmemmap_start = (unsigned long)head, vmemmap_end; unsigned long vmemmap_reuse; VM_WARN_ON_ONCE(!PageHuge(head)); if (!folio_test_hugetlb_vmemmap_optimized(folio)) return 0; 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, 0); } /** * 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; 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, VMEMMAP_REMAP_NO_TLB_FLUSH); 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(const struct hstate *h, const struct page *head) { if (HPageVmemmapOptimized((struct page *)head)) return false; if (!READ_ONCE(vmemmap_optimize_enabled)) return false; if (!hugetlb_vmemmap_optimizable(h)) return false; if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) { pmd_t *pmdp, pmd; struct page *vmemmap_page; unsigned long vaddr = (unsigned long)head; /* * Only the vmemmap page's vmemmap page can be self-hosted. * Walking the page tables to find the backing page of the * vmemmap page. */ pmdp = pmd_off_k(vaddr); /* * The READ_ONCE() is used to stabilize *pmdp in a register or * on the stack so that it will stop changing under the code. * The only concurrent operation where it can be changed is * split_vmemmap_huge_pmd() (*pmdp will be stable after this * operation). */ pmd = READ_ONCE(*pmdp); if (pmd_leaf(pmd)) vmemmap_page = pmd_page(pmd) + pte_index(vaddr); else vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr)); /* * 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 any vmemmap * page's vmemmap page if it is marked as VmemmapSelfHosted is * sufficient. * * [ hotplugged memory ] * [ section ][...][ section ] * [ vmemmap ][ usable memory ] * ^ | | | * +---+ | | * ^ | | * +-------+ | * ^ | * +-------------------------------------------+ */ if (PageVmemmapSelfHosted(vmemmap_page)) 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; struct page *head = &folio->page; unsigned long vmemmap_start = (unsigned long)head, vmemmap_end; unsigned long vmemmap_reuse; VM_WARN_ON_ONCE(!PageHuge(head)); if (!vmemmap_should_optimize(h, head)) return ret; static_branch_inc(&hugetlb_optimize_vmemmap_key); /* * 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, 0); free_vmemmap_page_list(&vmemmap_pages); } static int hugetlb_vmemmap_split(const struct hstate *h, struct page *head) { unsigned long vmemmap_start = (unsigned long)head, vmemmap_end; unsigned long vmemmap_reuse; if (!vmemmap_should_optimize(h, head)) 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); list_for_each_entry(folio, folio_list, lru) { int ret = hugetlb_vmemmap_split(h, &folio->page); /* * 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 = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_REMAP_NO_TLB_FLUSH); /* * 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, VMEMMAP_REMAP_NO_TLB_FLUSH); } } 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);