mm/hugetlb_vmemmap.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Optimize vmemmap pages associated with HugeTLB
   4  *
   5  * Copyright (c) 2020, Bytedance. All rights reserved.
   6  *
   7  *     Author: Muchun Song <songmuchun@bytedance.com>
   8  *
   9  * The struct page structures (page structs) are used to describe a physical
  10  * page frame. By default, there is a one-to-one mapping from a page frame to
  11  * it's corresponding page struct.
  12  *
  13  * HugeTLB pages consist of multiple base page size pages and is supported by
  14  * many architectures. See hugetlbpage.rst in the Documentation directory for
  15  * more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
  16  * are currently supported. Since the base page size on x86 is 4KB, a 2MB
  17  * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
  18  * 4096 base pages. For each base page, there is a corresponding page struct.
  19  *
  20  * Within the HugeTLB subsystem, only the first 4 page structs are used to
  21  * contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides
  22  * this upper limit. The only 'useful' information in the remaining page structs
  23  * is the compound_head field, and this field is the same for all tail pages.
  24  *
  25  * By removing redundant page structs for HugeTLB pages, memory can be returned
  26  * to the buddy allocator for other uses.
  27  *
  28  * Different architectures support different HugeTLB pages. For example, the
  29  * following table is the HugeTLB page size supported by x86 and arm64
  30  * architectures. Because arm64 supports 4k, 16k, and 64k base pages and
  31  * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
  32  * page.
  33  *
  34  * +--------------+-----------+-----------------------------------------------+
  35  * | Architecture | Page Size |                HugeTLB Page Size              |
  36  * +--------------+-----------+-----------+-----------+-----------+-----------+
  37  * |    x86-64    |    4KB    |    2MB    |    1GB    |           |           |
  38  * +--------------+-----------+-----------+-----------+-----------+-----------+
  39  * |              |    4KB    |   64KB    |    2MB    |    32MB   |    1GB    |
  40  * |              +-----------+-----------+-----------+-----------+-----------+
  41  * |    arm64     |   16KB    |    2MB    |   32MB    |     1GB   |           |
  42  * |              +-----------+-----------+-----------+-----------+-----------+
  43  * |              |   64KB    |    2MB    |  512MB    |    16GB   |           |
  44  * +--------------+-----------+-----------+-----------+-----------+-----------+
  45  *
  46  * When the system boot up, every HugeTLB page has more than one struct page
  47  * structs which size is (unit: pages):
  48  *
  49  *    struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
  50  *
  51  * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
  52  * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
  53  * relationship.
  54  *
  55  *    HugeTLB_Size = n * PAGE_SIZE
  56  *
  57  * Then,
  58  *
  59  *    struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
  60  *                = n * sizeof(struct page) / PAGE_SIZE
  61  *
  62  * We can use huge mapping at the pud/pmd level for the HugeTLB page.
  63  *
  64  * For the HugeTLB page of the pmd level mapping, then
  65  *
  66  *    struct_size = n * sizeof(struct page) / PAGE_SIZE
  67  *                = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
  68  *                = sizeof(struct page) / sizeof(pte_t)
  69  *                = 64 / 8
  70  *                = 8 (pages)
  71  *
  72  * Where n is how many pte entries which one page can contains. So the value of
  73  * n is (PAGE_SIZE / sizeof(pte_t)).
  74  *
  75  * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
  76  * is 8. And this optimization also applicable only when the size of struct page
  77  * is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
  78  * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
  79  * size of struct page structs of it is 8 page frames which size depends on the
  80  * size of the base page.
  81  *
  82  * For the HugeTLB page of the pud level mapping, then
  83  *
  84  *    struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
  85  *                = PAGE_SIZE / 8 * 8 (pages)
  86  *                = PAGE_SIZE (pages)
  87  *
  88  * Where the struct_size(pmd) is the size of the struct page structs of a
  89  * HugeTLB page of the pmd level mapping.
  90  *
  91  * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
  92  * HugeTLB page consists in 4096.
  93  *
  94  * Next, we take the pmd level mapping of the HugeTLB page as an example to
  95  * show the internal implementation of this optimization. There are 8 pages
  96  * struct page structs associated with a HugeTLB page which is pmd mapped.
  97  *
  98  * Here is how things look before optimization.
  99  *
 100  *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 101  * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
 102  * |           |                     |     0     | -------------> |     0     |
 103  * |           |                     +-----------+                +-----------+
 104  * |           |                     |     1     | -------------> |     1     |
 105  * |           |                     +-----------+                +-----------+
 106  * |           |                     |     2     | -------------> |     2     |
 107  * |           |                     +-----------+                +-----------+
 108  * |           |                     |     3     | -------------> |     3     |
 109  * |           |                     +-----------+                +-----------+
 110  * |           |                     |     4     | -------------> |     4     |
 111  * |    PMD    |                     +-----------+                +-----------+
 112  * |   level   |                     |     5     | -------------> |     5     |
 113  * |  mapping  |                     +-----------+                +-----------+
 114  * |           |                     |     6     | -------------> |     6     |
 115  * |           |                     +-----------+                +-----------+
 116  * |           |                     |     7     | -------------> |     7     |
 117  * |           |                     +-----------+                +-----------+
 118  * |           |
 119  * |           |
 120  * |           |
 121  * +-----------+
 122  *
 123  * The value of page->compound_head is the same for all tail pages. The first
 124  * page of page structs (page 0) associated with the HugeTLB page contains the 4
 125  * page structs necessary to describe the HugeTLB. The only use of the remaining
 126  * pages of page structs (page 1 to page 7) is to point to page->compound_head.
 127  * Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of page structs
 128  * will be used for each HugeTLB page. This will allow us to free the remaining
 129  * 7 pages to the buddy allocator.
 130  *
 131  * Here is how things look after remapping.
 132  *
 133  *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 134  * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
 135  * |           |                     |     0     | -------------> |     0     |
 136  * |           |                     +-----------+                +-----------+
 137  * |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
 138  * |           |                     +-----------+                  | | | | | |
 139  * |           |                     |     2     | -----------------+ | | | | |
 140  * |           |                     +-----------+                    | | | | |
 141  * |           |                     |     3     | -------------------+ | | | |
 142  * |           |                     +-----------+                      | | | |
 143  * |           |                     |     4     | ---------------------+ | | |
 144  * |    PMD    |                     +-----------+                        | | |
 145  * |   level   |                     |     5     | -----------------------+ | |
 146  * |  mapping  |                     +-----------+                          | |
 147  * |           |                     |     6     | -------------------------+ |
 148  * |           |                     +-----------+                            |
 149  * |           |                     |     7     | ---------------------------+
 150  * |           |                     +-----------+
 151  * |           |
 152  * |           |
 153  * |           |
 154  * +-----------+
 155  *
 156  * When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
 157  * vmemmap pages and restore the previous mapping relationship.
 158  *
 159  * For the HugeTLB page of the pud level mapping. It is similar to the former.
 160  * We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
 161  *
 162  * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
 163  * (e.g. aarch64) provides a contiguous bit in the translation table entries
 164  * that hints to the MMU to indicate that it is one of a contiguous set of
 165  * entries that can be cached in a single TLB entry.
 166  *
 167  * The contiguous bit is used to increase the mapping size at the pmd and pte
 168  * (last) level. So this type of HugeTLB page can be optimized only when its
 169  * size of the struct page structs is greater than 1 page.
 170  *
 171  * Notice: The head vmemmap page is not freed to the buddy allocator and all
 172  * tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
 173  * more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
 174  * associated with each HugeTLB page. The compound_head() can handle this
 175  * correctly (more details refer to the comment above compound_head()).
 176  */
 177 #define pr_fmt(fmt)     "HugeTLB: " fmt
 178
 179 #include "hugetlb_vmemmap.h"
 180
 181 /*
 182  * There are a lot of struct page structures associated with each HugeTLB page.
 183  * For tail pages, the value of compound_head is the same. So we can reuse first
 184  * page of head page structures. We map the virtual addresses of all the pages
 185  * of tail page structures to the head page struct, and then free these page
 186  * frames. Therefore, we need to reserve one pages as vmemmap areas.
 187  */
 188 #define RESERVE_VMEMMAP_NR              1U
 189 #define RESERVE_VMEMMAP_SIZE            (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 190
 191 DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON,
 192                         hugetlb_optimize_vmemmap_key);
 193 EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);
 194
 195 static int __init hugetlb_vmemmap_early_param(char *buf)
 196 {
 197         /* We cannot optimize if a "struct page" crosses page boundaries. */
 198         if (!is_power_of_2(sizeof(struct page))) {
 199                 pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
 200                 return 0;
 201         }
 202
 203         if (!buf)
 204                 return -EINVAL;
 205
 206         if (!strcmp(buf, "on"))
 207                 static_branch_enable(&hugetlb_optimize_vmemmap_key);
 208         else if (!strcmp(buf, "off"))
 209                 static_branch_disable(&hugetlb_optimize_vmemmap_key);
 210         else
 211                 return -EINVAL;
 212
 213         return 0;
 214 }
 215 early_param("hugetlb_free_vmemmap", hugetlb_vmemmap_early_param);
 216
 217 /*
 218  * Previously discarded vmemmap pages will be allocated and remapping
 219  * after this function returns zero.
 220  */
 221 int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
 222 {
 223         int ret;
 224         unsigned long vmemmap_addr = (unsigned long)head;
 225         unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
 226
 227         if (!HPageVmemmapOptimized(head))
 228                 return 0;
 229
 230         vmemmap_addr    += RESERVE_VMEMMAP_SIZE;
 231         vmemmap_pages   = hugetlb_optimize_vmemmap_pages(h);
 232         vmemmap_end     = vmemmap_addr + (vmemmap_pages << PAGE_SHIFT);
 233         vmemmap_reuse   = vmemmap_addr - PAGE_SIZE;
 234
 235         /*
 236          * The pages which the vmemmap virtual address range [@vmemmap_addr,
 237          * @vmemmap_end) are mapped to are freed to the buddy allocator, and
 238          * the range is mapped to the page which @vmemmap_reuse is mapped to.
 239          * When a HugeTLB page is freed to the buddy allocator, previously
 240          * discarded vmemmap pages must be allocated and remapping.
 241          */
 242         ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
 243                                   GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
 244         if (!ret)
 245                 ClearHPageVmemmapOptimized(head);
 246
 247         return ret;
 248 }
 249
 250 void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
 251 {
 252         unsigned long vmemmap_addr = (unsigned long)head;
 253         unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
 254
 255         vmemmap_pages = hugetlb_optimize_vmemmap_pages(h);
 256         if (!vmemmap_pages)
 257                 return;
 258
 259         vmemmap_addr    += RESERVE_VMEMMAP_SIZE;
 260         vmemmap_end     = vmemmap_addr + (vmemmap_pages << PAGE_SHIFT);
 261         vmemmap_reuse   = vmemmap_addr - PAGE_SIZE;
 262
 263         /*
 264          * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
 265          * to the page which @vmemmap_reuse is mapped to, then free the pages
 266          * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
 267          */
 268         if (!vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse))
 269                 SetHPageVmemmapOptimized(head);
 270 }
 271
 272 void __init hugetlb_vmemmap_init(struct hstate *h)
 273 {
 274         unsigned int nr_pages = pages_per_huge_page(h);
 275         unsigned int vmemmap_pages;
 276
 277         /*
 278          * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
 279          * page structs that can be used when CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP,
 280          * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
 281          */
 282         BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 283                      RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 284
 285         if (!hugetlb_optimize_vmemmap_enabled())
 286                 return;
 287
 288         vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
 289         /*
 290          * The head page is not to be freed to buddy allocator, the other tail
 291          * pages will map to the head page, so they can be freed.
 292          *
 293          * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
 294          * on some architectures (e.g. aarch64). See Documentation/arm64/
 295          * hugetlbpage.rst for more details.
 296          */
 297         if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
 298                 h->optimize_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
 299
 300         pr_info("can optimize %d vmemmap pages for %s\n",
 301                 h->optimize_vmemmap_pages, h->name);
 302 }