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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 | */ | |
171 | #include "hugetlb_vmemmap.h" | |
172 | ||
173 | /* | |
174 | * There are a lot of struct page structures associated with each HugeTLB page. | |
175 | * For tail pages, the value of compound_head is the same. So we can reuse first | |
176 | * page of tail page structures. We map the virtual addresses of the remaining | |
177 | * pages of tail page structures to the first tail page struct, and then free | |
178 | * these page frames. Therefore, we need to reserve two pages as vmemmap areas. | |
179 | */ | |
180 | #define RESERVE_VMEMMAP_NR 2U | |
181 | #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT) | |
182 | ||
f41f2ed4 MS |
183 | static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h) |
184 | { | |
185 | return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT; | |
186 | } | |
187 | ||
188 | void free_huge_page_vmemmap(struct hstate *h, struct page *head) | |
189 | { | |
190 | unsigned long vmemmap_addr = (unsigned long)head; | |
191 | unsigned long vmemmap_end, vmemmap_reuse; | |
192 | ||
193 | if (!free_vmemmap_pages_per_hpage(h)) | |
194 | return; | |
195 | ||
196 | vmemmap_addr += RESERVE_VMEMMAP_SIZE; | |
197 | vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); | |
198 | vmemmap_reuse = vmemmap_addr - PAGE_SIZE; | |
199 | ||
200 | /* | |
201 | * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end) | |
202 | * to the page which @vmemmap_reuse is mapped to, then free the pages | |
203 | * which the range [@vmemmap_addr, @vmemmap_end] is mapped to. | |
204 | */ | |
205 | vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse); | |
206 | } |