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 // 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/bootmem_info.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.
  */
 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 < PTRS_PER_PTE; 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(!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);
 
     /*
      * 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.
  */
 static 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.
  */
 static 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;
 }
 
 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);
 
 /**
  * hugetlb_vmemmap_restore - restore previously optimized (by
  *               hugetlb_vmemmap_optimize()) vmemmap pages which
  *               will be reallocated and remapped.
  * @h:      struct hstate.
  * @head:   the head page whose vmemmap pages will be restored.
  *
  * Return: %0 if @head's vmemmap pages have been reallocated and remapped,
  * negative error code otherwise.
  */
 int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 {
     int ret;
     unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
     unsigned long vmemmap_reuse;
 
     if (!HPageVmemmapOptimized(head))
         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,
                   GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
     if (!ret) {
         ClearHPageVmemmapOptimized(head);
         static_branch_dec(&hugetlb_optimize_vmemmap_key);
     }
 
     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 (!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;
 }
 
 /**
  * hugetlb_vmemmap_optimize - optimize @head page's vmemmap pages.
  * @h:      struct hstate.
  * @head:   the head page whose vmemmap pages will be optimized.
  *
  * This function only tries to optimize @head's vmemmap pages and does not
  * guarantee that the optimization will succeed after it returns. The caller
  * can use HPageVmemmapOptimized(@head) to detect if @head's vmemmap pages
  * have been optimized.
  */
 void hugetlb_vmemmap_optimize(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;
 
     static_branch_inc(&hugetlb_optimize_vmemmap_key);
 
     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, then free the pages
      * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
      */
     if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse))
         static_branch_dec(&hugetlb_optimize_vmemmap_key);
     else
         SetHPageVmemmapOptimized(head);
 }
 
 static struct ctl_table hugetlb_vmemmap_sysctls[] = {
     {
         .procname   = "hugetlb_optimize_vmemmap",
         .data       = &vmemmap_optimize_enabled,
         .maxlen     = sizeof(int),
         .mode       = 0644,
         .proc_handler   = proc_dobool,
     },
     { }
 };
 
 static int __init hugetlb_vmemmap_init(void)
 {
     /* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
     BUILD_BUG_ON(__NR_USED_SUBPAGE * sizeof(struct page) > HUGETLB_VMEMMAP_RESERVE_SIZE);
 
     if (IS_ENABLED(CONFIG_PROC_SYSCTL)) {
         const struct hstate *h;
 
         for_each_hstate(h) {
             if (hugetlb_vmemmap_optimizable(h)) {
                 register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
                 break;
             }
         }
     }
     return 0;
 }
 late_initcall(hugetlb_vmemmap_init);

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