1// SPDX-License-Identifier: GPL-2.0
2/*
3 * HugeTLB Vmemmap Optimization (HVO)
4 *
5 * Copyright (c) 2020, ByteDance. All rights reserved.
6 *
7 * Author: Muchun Song <songmuchun@bytedance.com>
8 *
9 * See Documentation/mm/vmemmap_dedup.rst
10 */
11#define pr_fmt(fmt) "HugeTLB: " fmt
12
13#include <linux/pgtable.h>
14#include <linux/moduleparam.h>
15#include <linux/bootmem_info.h>
16#include <linux/mmdebug.h>
17#include <asm/pgalloc.h>
18#include <asm/tlbflush.h>
19#include "hugetlb_vmemmap.h"
20
21/**
22 * struct vmemmap_remap_walk - walk vmemmap page table
23 *
24 * @remap_pte: called for each lowest-level entry (PTE).
25 * @nr_walked: the number of walked pte.
26 * @reuse_page: the page which is reused for the tail vmemmap pages.
27 * @reuse_addr: the virtual address of the @reuse_page page.
28 * @vmemmap_pages: the list head of the vmemmap pages that can be freed
29 * or is mapped from.
30 * @flags: used to modify behavior in vmemmap page table walking
31 * operations.
32 */
33struct vmemmap_remap_walk {
34 void (*remap_pte)(pte_t *pte, unsigned long addr,
35 struct vmemmap_remap_walk *walk);
36 unsigned long nr_walked;
37 struct page *reuse_page;
38 unsigned long reuse_addr;
39 struct list_head *vmemmap_pages;
40
41/* Skip the TLB flush when we split the PMD */
42#define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0)
43/* Skip the TLB flush when we remap the PTE */
44#define VMEMMAP_REMAP_NO_TLB_FLUSH BIT(1)
45 unsigned long flags;
46};
47
48static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start, bool flush)
49{
50 pmd_t __pmd;
51 int i;
52 unsigned long addr = start;
53 struct page *head;
54 pte_t *pgtable;
55
56 spin_lock(lock: &init_mm.page_table_lock);
57 head = pmd_leaf(pte: *pmd) ? pmd_page(*pmd) : NULL;
58 spin_unlock(lock: &init_mm.page_table_lock);
59
60 if (!head)
61 return 0;
62
63 pgtable = pte_alloc_one_kernel(mm: &init_mm);
64 if (!pgtable)
65 return -ENOMEM;
66
67 pmd_populate_kernel(mm: &init_mm, pmd: &__pmd, pte: pgtable);
68
69 for (i = 0; i < PTRS_PER_PTE; i++, addr += PAGE_SIZE) {
70 pte_t entry, *pte;
71 pgprot_t pgprot = PAGE_KERNEL;
72
73 entry = mk_pte(head + i, pgprot);
74 pte = pte_offset_kernel(pmd: &__pmd, address: addr);
75 set_pte_at(&init_mm, addr, pte, entry);
76 }
77
78 spin_lock(lock: &init_mm.page_table_lock);
79 if (likely(pmd_leaf(*pmd))) {
80 /*
81 * Higher order allocations from buddy allocator must be able to
82 * be treated as indepdenent small pages (as they can be freed
83 * individually).
84 */
85 if (!PageReserved(page: head))
86 split_page(page: head, order: get_order(PMD_SIZE));
87
88 /* Make pte visible before pmd. See comment in pmd_install(). */
89 smp_wmb();
90 pmd_populate_kernel(mm: &init_mm, pmd, pte: pgtable);
91 if (flush)
92 flush_tlb_kernel_range(start, end: start + PMD_SIZE);
93 } else {
94 pte_free_kernel(mm: &init_mm, pte: pgtable);
95 }
96 spin_unlock(lock: &init_mm.page_table_lock);
97
98 return 0;
99}
100
101static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
102 unsigned long end,
103 struct vmemmap_remap_walk *walk)
104{
105 pte_t *pte = pte_offset_kernel(pmd, address: addr);
106
107 /*
108 * The reuse_page is found 'first' in table walk before we start
109 * remapping (which is calling @walk->remap_pte).
110 */
111 if (!walk->reuse_page) {
112 walk->reuse_page = pte_page(ptep_get(pte));
113 /*
114 * Because the reuse address is part of the range that we are
115 * walking, skip the reuse address range.
116 */
117 addr += PAGE_SIZE;
118 pte++;
119 walk->nr_walked++;
120 }
121
122 for (; addr != end; addr += PAGE_SIZE, pte++) {
123 walk->remap_pte(pte, addr, walk);
124 walk->nr_walked++;
125 }
126}
127
128static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
129 unsigned long end,
130 struct vmemmap_remap_walk *walk)
131{
132 pmd_t *pmd;
133 unsigned long next;
134
135 pmd = pmd_offset(pud, address: addr);
136 do {
137 int ret;
138
139 ret = split_vmemmap_huge_pmd(pmd, start: addr & PMD_MASK,
140 flush: !(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH));
141 if (ret)
142 return ret;
143
144 next = pmd_addr_end(addr, end);
145
146 /*
147 * We are only splitting, not remapping the hugetlb vmemmap
148 * pages.
149 */
150 if (!walk->remap_pte)
151 continue;
152
153 vmemmap_pte_range(pmd, addr, end: next, walk);
154 } while (pmd++, addr = next, addr != end);
155
156 return 0;
157}
158
159static int vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
160 unsigned long end,
161 struct vmemmap_remap_walk *walk)
162{
163 pud_t *pud;
164 unsigned long next;
165
166 pud = pud_offset(p4d, address: addr);
167 do {
168 int ret;
169
170 next = pud_addr_end(addr, end);
171 ret = vmemmap_pmd_range(pud, addr, end: next, walk);
172 if (ret)
173 return ret;
174 } while (pud++, addr = next, addr != end);
175
176 return 0;
177}
178
179static int vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
180 unsigned long end,
181 struct vmemmap_remap_walk *walk)
182{
183 p4d_t *p4d;
184 unsigned long next;
185
186 p4d = p4d_offset(pgd, address: addr);
187 do {
188 int ret;
189
190 next = p4d_addr_end(addr, end);
191 ret = vmemmap_pud_range(p4d, addr, end: next, walk);
192 if (ret)
193 return ret;
194 } while (p4d++, addr = next, addr != end);
195
196 return 0;
197}
198
199static int vmemmap_remap_range(unsigned long start, unsigned long end,
200 struct vmemmap_remap_walk *walk)
201{
202 unsigned long addr = start;
203 unsigned long next;
204 pgd_t *pgd;
205
206 VM_BUG_ON(!PAGE_ALIGNED(start));
207 VM_BUG_ON(!PAGE_ALIGNED(end));
208
209 pgd = pgd_offset_k(addr);
210 do {
211 int ret;
212
213 next = pgd_addr_end(addr, end);
214 ret = vmemmap_p4d_range(pgd, addr, end: next, walk);
215 if (ret)
216 return ret;
217 } while (pgd++, addr = next, addr != end);
218
219 if (walk->remap_pte && !(walk->flags & VMEMMAP_REMAP_NO_TLB_FLUSH))
220 flush_tlb_kernel_range(start, end);
221
222 return 0;
223}
224
225/*
226 * Free a vmemmap page. A vmemmap page can be allocated from the memblock
227 * allocator or buddy allocator. If the PG_reserved flag is set, it means
228 * that it allocated from the memblock allocator, just free it via the
229 * free_bootmem_page(). Otherwise, use __free_page().
230 */
231static inline void free_vmemmap_page(struct page *page)
232{
233 if (PageReserved(page))
234 free_bootmem_page(page);
235 else
236 __free_page(page);
237}
238
239/* Free a list of the vmemmap pages */
240static void free_vmemmap_page_list(struct list_head *list)
241{
242 struct page *page, *next;
243
244 list_for_each_entry_safe(page, next, list, lru)
245 free_vmemmap_page(page);
246}
247
248static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
249 struct vmemmap_remap_walk *walk)
250{
251 /*
252 * Remap the tail pages as read-only to catch illegal write operation
253 * to the tail pages.
254 */
255 pgprot_t pgprot = PAGE_KERNEL_RO;
256 struct page *page = pte_page(ptep_get(pte));
257 pte_t entry;
258
259 /* Remapping the head page requires r/w */
260 if (unlikely(addr == walk->reuse_addr)) {
261 pgprot = PAGE_KERNEL;
262 list_del(entry: &walk->reuse_page->lru);
263
264 /*
265 * Makes sure that preceding stores to the page contents from
266 * vmemmap_remap_free() become visible before the set_pte_at()
267 * write.
268 */
269 smp_wmb();
270 }
271
272 entry = mk_pte(walk->reuse_page, pgprot);
273 list_add(new: &page->lru, head: walk->vmemmap_pages);
274 set_pte_at(&init_mm, addr, pte, entry);
275}
276
277/*
278 * How many struct page structs need to be reset. When we reuse the head
279 * struct page, the special metadata (e.g. page->flags or page->mapping)
280 * cannot copy to the tail struct page structs. The invalid value will be
281 * checked in the free_tail_page_prepare(). In order to avoid the message
282 * of "corrupted mapping in tail page". We need to reset at least 3 (one
283 * head struct page struct and two tail struct page structs) struct page
284 * structs.
285 */
286#define NR_RESET_STRUCT_PAGE 3
287
288static inline void reset_struct_pages(struct page *start)
289{
290 struct page *from = start + NR_RESET_STRUCT_PAGE;
291
292 BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
293 memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE);
294}
295
296static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
297 struct vmemmap_remap_walk *walk)
298{
299 pgprot_t pgprot = PAGE_KERNEL;
300 struct page *page;
301 void *to;
302
303 BUG_ON(pte_page(ptep_get(pte)) != walk->reuse_page);
304
305 page = list_first_entry(walk->vmemmap_pages, struct page, lru);
306 list_del(entry: &page->lru);
307 to = page_to_virt(page);
308 copy_page(to, from: (void *)walk->reuse_addr);
309 reset_struct_pages(start: to);
310
311 /*
312 * Makes sure that preceding stores to the page contents become visible
313 * before the set_pte_at() write.
314 */
315 smp_wmb();
316 set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
317}
318
319/**
320 * vmemmap_remap_split - split the vmemmap virtual address range [@start, @end)
321 * backing PMDs of the directmap into PTEs
322 * @start: start address of the vmemmap virtual address range that we want
323 * to remap.
324 * @end: end address of the vmemmap virtual address range that we want to
325 * remap.
326 * @reuse: reuse address.
327 *
328 * Return: %0 on success, negative error code otherwise.
329 */
330static int vmemmap_remap_split(unsigned long start, unsigned long end,
331 unsigned long reuse)
332{
333 int ret;
334 struct vmemmap_remap_walk walk = {
335 .remap_pte = NULL,
336 .flags = VMEMMAP_SPLIT_NO_TLB_FLUSH,
337 };
338
339 /* See the comment in the vmemmap_remap_free(). */
340 BUG_ON(start - reuse != PAGE_SIZE);
341
342 mmap_read_lock(mm: &init_mm);
343 ret = vmemmap_remap_range(start: reuse, end, walk: &walk);
344 mmap_read_unlock(mm: &init_mm);
345
346 return ret;
347}
348
349/**
350 * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
351 * to the page which @reuse is mapped to, then free vmemmap
352 * which the range are mapped to.
353 * @start: start address of the vmemmap virtual address range that we want
354 * to remap.
355 * @end: end address of the vmemmap virtual address range that we want to
356 * remap.
357 * @reuse: reuse address.
358 * @vmemmap_pages: list to deposit vmemmap pages to be freed. It is callers
359 * responsibility to free pages.
360 * @flags: modifications to vmemmap_remap_walk flags
361 *
362 * Return: %0 on success, negative error code otherwise.
363 */
364static int vmemmap_remap_free(unsigned long start, unsigned long end,
365 unsigned long reuse,
366 struct list_head *vmemmap_pages,
367 unsigned long flags)
368{
369 int ret;
370 struct vmemmap_remap_walk walk = {
371 .remap_pte = vmemmap_remap_pte,
372 .reuse_addr = reuse,
373 .vmemmap_pages = vmemmap_pages,
374 .flags = flags,
375 };
376 int nid = page_to_nid(page: (struct page *)reuse);
377 gfp_t gfp_mask = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
378
379 /*
380 * Allocate a new head vmemmap page to avoid breaking a contiguous
381 * block of struct page memory when freeing it back to page allocator
382 * in free_vmemmap_page_list(). This will allow the likely contiguous
383 * struct page backing memory to be kept contiguous and allowing for
384 * more allocations of hugepages. Fallback to the currently
385 * mapped head page in case should it fail to allocate.
386 */
387 walk.reuse_page = alloc_pages_node(nid, gfp_mask, order: 0);
388 if (walk.reuse_page) {
389 copy_page(page_to_virt(walk.reuse_page),
390 from: (void *)walk.reuse_addr);
391 list_add(new: &walk.reuse_page->lru, head: vmemmap_pages);
392 }
393
394 /*
395 * In order to make remapping routine most efficient for the huge pages,
396 * the routine of vmemmap page table walking has the following rules
397 * (see more details from the vmemmap_pte_range()):
398 *
399 * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
400 * should be continuous.
401 * - The @reuse address is part of the range [@reuse, @end) that we are
402 * walking which is passed to vmemmap_remap_range().
403 * - The @reuse address is the first in the complete range.
404 *
405 * So we need to make sure that @start and @reuse meet the above rules.
406 */
407 BUG_ON(start - reuse != PAGE_SIZE);
408
409 mmap_read_lock(mm: &init_mm);
410 ret = vmemmap_remap_range(start: reuse, end, walk: &walk);
411 if (ret && walk.nr_walked) {
412 end = reuse + walk.nr_walked * PAGE_SIZE;
413 /*
414 * vmemmap_pages contains pages from the previous
415 * vmemmap_remap_range call which failed. These
416 * are pages which were removed from the vmemmap.
417 * They will be restored in the following call.
418 */
419 walk = (struct vmemmap_remap_walk) {
420 .remap_pte = vmemmap_restore_pte,
421 .reuse_addr = reuse,
422 .vmemmap_pages = vmemmap_pages,
423 .flags = 0,
424 };
425
426 vmemmap_remap_range(start: reuse, end, walk: &walk);
427 }
428 mmap_read_unlock(mm: &init_mm);
429
430 return ret;
431}
432
433static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
434 struct list_head *list)
435{
436 gfp_t gfp_mask = GFP_KERNEL | __GFP_RETRY_MAYFAIL;
437 unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
438 int nid = page_to_nid(page: (struct page *)start);
439 struct page *page, *next;
440
441 while (nr_pages--) {
442 page = alloc_pages_node(nid, gfp_mask, order: 0);
443 if (!page)
444 goto out;
445 list_add(new: &page->lru, head: list);
446 }
447
448 return 0;
449out:
450 list_for_each_entry_safe(page, next, list, lru)
451 __free_page(page);
452 return -ENOMEM;
453}
454
455/**
456 * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
457 * to the page which is from the @vmemmap_pages
458 * respectively.
459 * @start: start address of the vmemmap virtual address range that we want
460 * to remap.
461 * @end: end address of the vmemmap virtual address range that we want to
462 * remap.
463 * @reuse: reuse address.
464 * @flags: modifications to vmemmap_remap_walk flags
465 *
466 * Return: %0 on success, negative error code otherwise.
467 */
468static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
469 unsigned long reuse, unsigned long flags)
470{
471 LIST_HEAD(vmemmap_pages);
472 struct vmemmap_remap_walk walk = {
473 .remap_pte = vmemmap_restore_pte,
474 .reuse_addr = reuse,
475 .vmemmap_pages = &vmemmap_pages,
476 .flags = flags,
477 };
478
479 /* See the comment in the vmemmap_remap_free(). */
480 BUG_ON(start - reuse != PAGE_SIZE);
481
482 if (alloc_vmemmap_page_list(start, end, list: &vmemmap_pages))
483 return -ENOMEM;
484
485 mmap_read_lock(mm: &init_mm);
486 vmemmap_remap_range(start: reuse, end, walk: &walk);
487 mmap_read_unlock(mm: &init_mm);
488
489 return 0;
490}
491
492DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
493EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);
494
495static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
496core_param(hugetlb_free_vmemmap, vmemmap_optimize_enabled, bool, 0);
497
498static int __hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio, unsigned long flags)
499{
500 int ret;
501 struct page *head = &folio->page;
502 unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
503 unsigned long vmemmap_reuse;
504
505 VM_WARN_ON_ONCE(!PageHuge(head));
506 if (!folio_test_hugetlb_vmemmap_optimized(folio))
507 return 0;
508
509 vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
510 vmemmap_reuse = vmemmap_start;
511 vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
512
513 /*
514 * The pages which the vmemmap virtual address range [@vmemmap_start,
515 * @vmemmap_end) are mapped to are freed to the buddy allocator, and
516 * the range is mapped to the page which @vmemmap_reuse is mapped to.
517 * When a HugeTLB page is freed to the buddy allocator, previously
518 * discarded vmemmap pages must be allocated and remapping.
519 */
520 ret = vmemmap_remap_alloc(start: vmemmap_start, end: vmemmap_end, reuse: vmemmap_reuse, flags);
521 if (!ret) {
522 folio_clear_hugetlb_vmemmap_optimized(folio);
523 static_branch_dec(&hugetlb_optimize_vmemmap_key);
524 }
525
526 return ret;
527}
528
529/**
530 * hugetlb_vmemmap_restore_folio - restore previously optimized (by
531 * hugetlb_vmemmap_optimize_folio()) vmemmap pages which
532 * will be reallocated and remapped.
533 * @h: struct hstate.
534 * @folio: the folio whose vmemmap pages will be restored.
535 *
536 * Return: %0 if @folio's vmemmap pages have been reallocated and remapped,
537 * negative error code otherwise.
538 */
539int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
540{
541 return __hugetlb_vmemmap_restore_folio(h, folio, flags: 0);
542}
543
544/**
545 * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
546 * @h: hstate.
547 * @folio_list: list of folios.
548 * @non_hvo_folios: Output list of folios for which vmemmap exists.
549 *
550 * Return: number of folios for which vmemmap was restored, or an error code
551 * if an error was encountered restoring vmemmap for a folio.
552 * Folios that have vmemmap are moved to the non_hvo_folios
553 * list. Processing of entries stops when the first error is
554 * encountered. The folio that experienced the error and all
555 * non-processed folios will remain on folio_list.
556 */
557long hugetlb_vmemmap_restore_folios(const struct hstate *h,
558 struct list_head *folio_list,
559 struct list_head *non_hvo_folios)
560{
561 struct folio *folio, *t_folio;
562 long restored = 0;
563 long ret = 0;
564
565 list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
566 if (folio_test_hugetlb_vmemmap_optimized(folio)) {
567 ret = __hugetlb_vmemmap_restore_folio(h, folio,
568 VMEMMAP_REMAP_NO_TLB_FLUSH);
569 if (ret)
570 break;
571 restored++;
572 }
573
574 /* Add non-optimized folios to output list */
575 list_move(list: &folio->lru, head: non_hvo_folios);
576 }
577
578 if (restored)
579 flush_tlb_all();
580 if (!ret)
581 ret = restored;
582 return ret;
583}
584
585/* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
586static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
587{
588 if (HPageVmemmapOptimized(page: (struct page *)head))
589 return false;
590
591 if (!READ_ONCE(vmemmap_optimize_enabled))
592 return false;
593
594 if (!hugetlb_vmemmap_optimizable(h))
595 return false;
596
597 if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
598 pmd_t *pmdp, pmd;
599 struct page *vmemmap_page;
600 unsigned long vaddr = (unsigned long)head;
601
602 /*
603 * Only the vmemmap page's vmemmap page can be self-hosted.
604 * Walking the page tables to find the backing page of the
605 * vmemmap page.
606 */
607 pmdp = pmd_off_k(va: vaddr);
608 /*
609 * The READ_ONCE() is used to stabilize *pmdp in a register or
610 * on the stack so that it will stop changing under the code.
611 * The only concurrent operation where it can be changed is
612 * split_vmemmap_huge_pmd() (*pmdp will be stable after this
613 * operation).
614 */
615 pmd = READ_ONCE(*pmdp);
616 if (pmd_leaf(pte: pmd))
617 vmemmap_page = pmd_page(pmd) + pte_index(address: vaddr);
618 else
619 vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr));
620 /*
621 * Due to HugeTLB alignment requirements and the vmemmap pages
622 * being at the start of the hotplugged memory region in
623 * memory_hotplug.memmap_on_memory case. Checking any vmemmap
624 * page's vmemmap page if it is marked as VmemmapSelfHosted is
625 * sufficient.
626 *
627 * [ hotplugged memory ]
628 * [ section ][...][ section ]
629 * [ vmemmap ][ usable memory ]
630 * ^ | | |
631 * +---+ | |
632 * ^ | |
633 * +-------+ |
634 * ^ |
635 * +-------------------------------------------+
636 */
637 if (PageVmemmapSelfHosted(page: vmemmap_page))
638 return false;
639 }
640
641 return true;
642}
643
644static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
645 struct folio *folio,
646 struct list_head *vmemmap_pages,
647 unsigned long flags)
648{
649 int ret = 0;
650 struct page *head = &folio->page;
651 unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
652 unsigned long vmemmap_reuse;
653
654 VM_WARN_ON_ONCE(!PageHuge(head));
655 if (!vmemmap_should_optimize(h, head))
656 return ret;
657
658 static_branch_inc(&hugetlb_optimize_vmemmap_key);
659 /*
660 * Very Subtle
661 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
662 * immediately after remapping. As a result, subsequent accesses
663 * and modifications to struct pages associated with the hugetlb
664 * page could be to the OLD struct pages. Set the vmemmap optimized
665 * flag here so that it is copied to the new head page. This keeps
666 * the old and new struct pages in sync.
667 * If there is an error during optimization, we will immediately FLUSH
668 * the TLB and clear the flag below.
669 */
670 folio_set_hugetlb_vmemmap_optimized(folio);
671
672 vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
673 vmemmap_reuse = vmemmap_start;
674 vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
675
676 /*
677 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
678 * to the page which @vmemmap_reuse is mapped to. Add pages previously
679 * mapping the range to vmemmap_pages list so that they can be freed by
680 * the caller.
681 */
682 ret = vmemmap_remap_free(start: vmemmap_start, end: vmemmap_end, reuse: vmemmap_reuse,
683 vmemmap_pages, flags);
684 if (ret) {
685 static_branch_dec(&hugetlb_optimize_vmemmap_key);
686 folio_clear_hugetlb_vmemmap_optimized(folio);
687 }
688
689 return ret;
690}
691
692/**
693 * hugetlb_vmemmap_optimize_folio - optimize @folio's vmemmap pages.
694 * @h: struct hstate.
695 * @folio: the folio whose vmemmap pages will be optimized.
696 *
697 * This function only tries to optimize @folio's vmemmap pages and does not
698 * guarantee that the optimization will succeed after it returns. The caller
699 * can use folio_test_hugetlb_vmemmap_optimized(@folio) to detect if @folio's
700 * vmemmap pages have been optimized.
701 */
702void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
703{
704 LIST_HEAD(vmemmap_pages);
705
706 __hugetlb_vmemmap_optimize_folio(h, folio, vmemmap_pages: &vmemmap_pages, flags: 0);
707 free_vmemmap_page_list(list: &vmemmap_pages);
708}
709
710static int hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
711{
712 unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
713 unsigned long vmemmap_reuse;
714
715 if (!vmemmap_should_optimize(h, head))
716 return 0;
717
718 vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
719 vmemmap_reuse = vmemmap_start;
720 vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
721
722 /*
723 * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
724 * @vmemmap_end]
725 */
726 return vmemmap_remap_split(start: vmemmap_start, end: vmemmap_end, reuse: vmemmap_reuse);
727}
728
729void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
730{
731 struct folio *folio;
732 LIST_HEAD(vmemmap_pages);
733
734 list_for_each_entry(folio, folio_list, lru) {
735 int ret = hugetlb_vmemmap_split(h, head: &folio->page);
736
737 /*
738 * Spliting the PMD requires allocating a page, thus lets fail
739 * early once we encounter the first OOM. No point in retrying
740 * as it can be dynamically done on remap with the memory
741 * we get back from the vmemmap deduplication.
742 */
743 if (ret == -ENOMEM)
744 break;
745 }
746
747 flush_tlb_all();
748
749 list_for_each_entry(folio, folio_list, lru) {
750 int ret = __hugetlb_vmemmap_optimize_folio(h, folio,
751 vmemmap_pages: &vmemmap_pages,
752 VMEMMAP_REMAP_NO_TLB_FLUSH);
753
754 /*
755 * Pages to be freed may have been accumulated. If we
756 * encounter an ENOMEM, free what we have and try again.
757 * This can occur in the case that both spliting fails
758 * halfway and head page allocation also failed. In this
759 * case __hugetlb_vmemmap_optimize_folio() would free memory
760 * allowing more vmemmap remaps to occur.
761 */
762 if (ret == -ENOMEM && !list_empty(head: &vmemmap_pages)) {
763 flush_tlb_all();
764 free_vmemmap_page_list(list: &vmemmap_pages);
765 INIT_LIST_HEAD(list: &vmemmap_pages);
766 __hugetlb_vmemmap_optimize_folio(h, folio,
767 vmemmap_pages: &vmemmap_pages,
768 VMEMMAP_REMAP_NO_TLB_FLUSH);
769 }
770 }
771
772 flush_tlb_all();
773 free_vmemmap_page_list(list: &vmemmap_pages);
774}
775
776static struct ctl_table hugetlb_vmemmap_sysctls[] = {
777 {
778 .procname = "hugetlb_optimize_vmemmap",
779 .data = &vmemmap_optimize_enabled,
780 .maxlen = sizeof(vmemmap_optimize_enabled),
781 .mode = 0644,
782 .proc_handler = proc_dobool,
783 },
784 { }
785};
786
787static int __init hugetlb_vmemmap_init(void)
788{
789 const struct hstate *h;
790
791 /* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
792 BUILD_BUG_ON(__NR_USED_SUBPAGE > HUGETLB_VMEMMAP_RESERVE_PAGES);
793
794 for_each_hstate(h) {
795 if (hugetlb_vmemmap_optimizable(h)) {
796 register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
797 break;
798 }
799 }
800 return 0;
801}
802late_initcall(hugetlb_vmemmap_init);
803

source code of linux/mm/hugetlb_vmemmap.c