Lines Matching +full:page +full:- +full:size
1 .. SPDX-License-Identifier: GPL-2.0
12 The ``struct page`` structures are used to describe a physical page frame. By
13 default, there is a one-to-one mapping from a page frame to it's corresponding
14 ``struct page``.
16 HugeTLB pages consist of multiple base page size pages and is supported by many
17 architectures. See Documentation/admin-guide/mm/hugetlbpage.rst for more
18 details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB are
19 currently supported. Since the base page size on x86 is 4KB, a 2MB HugeTLB page
20 consists of 512 base pages and a 1GB HugeTLB page consists of 4096 base pages.
21 For each base page, there is a corresponding ``struct page``.
23 Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to
24 contain unique information about a HugeTLB page. ``__NR_USED_SUBPAGE`` provides
25 this upper limit. The only 'useful' information in the remaining ``struct page``
28 By removing redundant ``struct page`` for HugeTLB pages, memory can be returned
32 following table is the HugeTLB page size supported by x86 and arm64
35 page.
37 +--------------+-----------+-----------------------------------------------+
38 | Architecture | Page Size | HugeTLB Page Size |
39 +--------------+-----------+-----------+-----------+-----------+-----------+
40 | x86-64 | 4KB | 2MB | 1GB | | |
41 +--------------+-----------+-----------+-----------+-----------+-----------+
43 | +-----------+-----------+-----------+-----------+-----------+
45 | +-----------+-----------+-----------+-----------+-----------+
47 +--------------+-----------+-----------+-----------+-----------+-----------+
49 When the system boot up, every HugeTLB page has more than one ``struct page``
50 structs which size is (unit: pages)::
52 struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
54 Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
55 of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
62 struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
63 = n * sizeof(struct page) / PAGE_SIZE
65 We can use huge mapping at the pud/pmd level for the HugeTLB page.
67 For the HugeTLB page of the pmd level mapping, then::
69 struct_size = n * sizeof(struct page) / PAGE_SIZE
70 = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
71 = sizeof(struct page) / sizeof(pte_t)
75 Where n is how many pte entries which one page can contains. So the value of
78 This optimization only supports 64-bit system, so the value of sizeof(pte_t)
79 is 8. And this optimization also applicable only when the size of ``struct page``
80 is a power of two. In most cases, the size of ``struct page`` is 64 bytes (e.g.
81 x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
82 size of ``struct page`` structs of it is 8 page frames which size depends on the
83 size of the base page.
85 For the HugeTLB page of the pud level mapping, then::
91 Where the struct_size(pmd) is the size of the ``struct page`` structs of a
92 HugeTLB page of the pmd level mapping.
94 E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
95 HugeTLB page consists in 4096.
97 Next, we take the pmd level mapping of the HugeTLB page as an example to
99 ``struct page`` structs associated with a HugeTLB page which is pmd mapped.
103 HugeTLB struct pages(8 pages) page frame(8 pages)
104 +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
105 | | | 0 | -------------> | 0 |
106 | | +-----------+ +-----------+
107 | | | 1 | -------------> | 1 |
108 | | +-----------+ +-----------+
109 | | | 2 | -------------> | 2 |
110 | | +-----------+ +-----------+
111 | | | 3 | -------------> | 3 |
112 | | +-----------+ +-----------+
113 | | | 4 | -------------> | 4 |
114 | PMD | +-----------+ +-----------+
115 | level | | 5 | -------------> | 5 |
116 | mapping | +-----------+ +-----------+
117 | | | 6 | -------------> | 6 |
118 | | +-----------+ +-----------+
119 | | | 7 | -------------> | 7 |
120 | | +-----------+ +-----------+
124 +-----------+
126 The value of page->compound_head is the same for all tail pages. The first
127 page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4
128 ``struct page`` necessary to describe the HugeTLB. The only use of the remaining
129 pages of ``struct page`` (page 1 to page 7) is to point to page->compound_head.
130 Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page``
131 will be used for each HugeTLB page. This will allow us to free the remaining
136 HugeTLB struct pages(8 pages) page frame(8 pages)
137 +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
138 | | | 0 | -------------> | 0 |
139 | | +-----------+ +-----------+
140 | | | 1 | ---------------^ ^ ^ ^ ^ ^ ^
141 | | +-----------+ | | | | | |
142 | | | 2 | -----------------+ | | | | |
143 | | +-----------+ | | | | |
144 | | | 3 | -------------------+ | | | |
145 | | +-----------+ | | | |
146 | | | 4 | ---------------------+ | | |
147 | PMD | +-----------+ | | |
148 | level | | 5 | -----------------------+ | |
149 | mapping | +-----------+ | |
150 | | | 6 | -------------------------+ |
151 | | +-----------+ |
152 | | | 7 | ---------------------------+
153 | | +-----------+
157 +-----------+
162 For the HugeTLB page of the pud level mapping. It is similar to the former.
163 We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
165 Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
170 The contiguous bit is used to increase the mapping size at the pmd and pte
171 (last) level. So this type of HugeTLB page can be optimized only when its
172 size of the ``struct page`` structs is greater than **1** page.
174 Notice: The head vmemmap page is not freed to the buddy allocator and all
175 tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
176 more than one ``struct page`` struct with ``PG_head`` (e.g. 8 per 2 MB HugeTLB
177 page) associated with each HugeTLB page. The ``compound_head()`` can handle
178 this correctly. There is only **one** head ``struct page``, the tail
179 ``struct page`` with ``PG_head`` are fake head ``struct page``. We need an
180 approach to distinguish between those two different types of ``struct page`` so
181 that ``compound_head()`` can return the real head ``struct page`` when the
182 parameter is the tail ``struct page`` but with ``PG_head``. The following code
183 snippet describes how to distinguish between real and fake head ``struct page``.
185 .. code-block:: c
187 if (test_bit(PG_head, &page->flags)) {
188 unsigned long head = READ_ONCE(page[1].compound_head);
191 if (head == (unsigned long)page + 1)
192 /* head struct page */
194 /* tail struct page */
196 /* head struct page */
200 We can safely access the field of the **page[1]** with ``PG_head`` because the
201 page is a compound page composed with at least two contiguous pages.
207 The device-dax interface uses the same tail deduplication technique explained
211 The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64),
216 It only use 3 ``struct page`` for storing all information as opposed
219 There's no remapping of vmemmap given that device-dax memory is not part of
220 System RAM ranges initialized at boot. Thus the tail page deduplication
222 the head vmemmap page representing, whereas device-dax reuses the tail
223 vmemmap page. This results in only half of the savings compared to HugeTLB.
225 Deduplicated tail pages are not mapped read-only.
227 Here's how things look like on device-dax after the sections are populated::
229 +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
230 | | | 0 | -------------> | 0 |
231 | | +-----------+ +-----------+
232 | | | 1 | -------------> | 1 |
233 | | +-----------+ +-----------+
234 | | | 2 | ----------------^ ^ ^ ^ ^ ^
235 | | +-----------+ | | | | |
236 | | | 3 | ------------------+ | | | |
237 | | +-----------+ | | | |
238 | | | 4 | --------------------+ | | |
239 | PMD | +-----------+ | | |
240 | level | | 5 | ----------------------+ | |
241 | mapping | +-----------+ | |
242 | | | 6 | ------------------------+ |
243 | | +-----------+ |
244 | | | 7 | --------------------------+
245 | | +-----------+
249 +-----------+