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f41f2ed4 MS |
1 | // SPDX-License-Identifier: GPL-2.0 |
2 | /* | |
3 | * Free some vmemmap pages of 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 2 to 7 to page 1. Only 2 pages of page structs | |
128 | * will be used for each HugeTLB page. This will allow us to free the remaining | |
129 | * 6 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 | -------------> | 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 6 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 - 2) 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 2 pages. | |
170 | */ | |
e9fdff87 MS |
171 | #define pr_fmt(fmt) "HugeTLB: " fmt |
172 | ||
f41f2ed4 MS |
173 | #include "hugetlb_vmemmap.h" |
174 | ||
175 | /* | |
176 | * There are a lot of struct page structures associated with each HugeTLB page. | |
177 | * For tail pages, the value of compound_head is the same. So we can reuse first | |
178 | * page of tail page structures. We map the virtual addresses of the remaining | |
179 | * pages of tail page structures to the first tail page struct, and then free | |
180 | * these page frames. Therefore, we need to reserve two pages as vmemmap areas. | |
181 | */ | |
182 | #define RESERVE_VMEMMAP_NR 2U | |
183 | #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT) | |
184 | ||
e6d41f12 | 185 | bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON); |
e9fdff87 MS |
186 | |
187 | static int __init early_hugetlb_free_vmemmap_param(char *buf) | |
188 | { | |
189 | /* We cannot optimize if a "struct page" crosses page boundaries. */ | |
190 | if ((!is_power_of_2(sizeof(struct page)))) { | |
191 | pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n"); | |
192 | return 0; | |
193 | } | |
194 | ||
195 | if (!buf) | |
196 | return -EINVAL; | |
197 | ||
198 | if (!strcmp(buf, "on")) | |
199 | hugetlb_free_vmemmap_enabled = true; | |
e6d41f12 MS |
200 | else if (!strcmp(buf, "off")) |
201 | hugetlb_free_vmemmap_enabled = false; | |
202 | else | |
e9fdff87 MS |
203 | return -EINVAL; |
204 | ||
205 | return 0; | |
206 | } | |
207 | early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param); | |
208 | ||
f41f2ed4 MS |
209 | static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h) |
210 | { | |
211 | return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT; | |
212 | } | |
213 | ||
ad2fa371 MS |
214 | /* |
215 | * Previously discarded vmemmap pages will be allocated and remapping | |
216 | * after this function returns zero. | |
217 | */ | |
218 | int alloc_huge_page_vmemmap(struct hstate *h, struct page *head) | |
219 | { | |
220 | int ret; | |
221 | unsigned long vmemmap_addr = (unsigned long)head; | |
222 | unsigned long vmemmap_end, vmemmap_reuse; | |
223 | ||
224 | if (!HPageVmemmapOptimized(head)) | |
225 | return 0; | |
226 | ||
227 | vmemmap_addr += RESERVE_VMEMMAP_SIZE; | |
228 | vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); | |
229 | vmemmap_reuse = vmemmap_addr - PAGE_SIZE; | |
230 | /* | |
231 | * The pages which the vmemmap virtual address range [@vmemmap_addr, | |
232 | * @vmemmap_end) are mapped to are freed to the buddy allocator, and | |
233 | * the range is mapped to the page which @vmemmap_reuse is mapped to. | |
234 | * When a HugeTLB page is freed to the buddy allocator, previously | |
235 | * discarded vmemmap pages must be allocated and remapping. | |
236 | */ | |
237 | ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, | |
238 | GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE); | |
239 | ||
240 | if (!ret) | |
241 | ClearHPageVmemmapOptimized(head); | |
242 | ||
243 | return ret; | |
244 | } | |
245 | ||
f41f2ed4 MS |
246 | void free_huge_page_vmemmap(struct hstate *h, struct page *head) |
247 | { | |
248 | unsigned long vmemmap_addr = (unsigned long)head; | |
249 | unsigned long vmemmap_end, vmemmap_reuse; | |
250 | ||
251 | if (!free_vmemmap_pages_per_hpage(h)) | |
252 | return; | |
253 | ||
254 | vmemmap_addr += RESERVE_VMEMMAP_SIZE; | |
255 | vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); | |
256 | vmemmap_reuse = vmemmap_addr - PAGE_SIZE; | |
257 | ||
258 | /* | |
259 | * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end) | |
260 | * to the page which @vmemmap_reuse is mapped to, then free the pages | |
261 | * which the range [@vmemmap_addr, @vmemmap_end] is mapped to. | |
262 | */ | |
3bc2b6a7 MS |
263 | if (!vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse)) |
264 | SetHPageVmemmapOptimized(head); | |
f41f2ed4 | 265 | } |
77490587 MS |
266 | |
267 | void __init hugetlb_vmemmap_init(struct hstate *h) | |
268 | { | |
269 | unsigned int nr_pages = pages_per_huge_page(h); | |
270 | unsigned int vmemmap_pages; | |
271 | ||
272 | /* | |
273 | * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct | |
274 | * page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, | |
275 | * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page. | |
276 | */ | |
277 | BUILD_BUG_ON(__NR_USED_SUBPAGE >= | |
278 | RESERVE_VMEMMAP_SIZE / sizeof(struct page)); | |
279 | ||
280 | if (!hugetlb_free_vmemmap_enabled) | |
281 | return; | |
282 | ||
283 | vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT; | |
284 | /* | |
285 | * The head page and the first tail page are not to be freed to buddy | |
286 | * allocator, the other pages will map to the first tail page, so they | |
287 | * can be freed. | |
288 | * | |
289 | * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true | |
290 | * on some architectures (e.g. aarch64). See Documentation/arm64/ | |
291 | * hugetlbpage.rst for more details. | |
292 | */ | |
293 | if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR)) | |
294 | h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR; | |
295 | ||
296 | pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages, | |
297 | h->name); | |
298 | } |