1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4A vmemmap diet for HugeTLB and Device DAX
5=========================================
6
7HugeTLB
8=======
9
10This section is to explain how HugeTLB Vmemmap Optimization (HVO) works.
11
12The ``struct page`` structures are used to describe a physical page frame. By
13default, there is a one-to-one mapping from a page frame to it's corresponding
14``struct page``.
15
16HugeTLB pages consist of multiple base page size pages and is supported by many
17architectures. See Documentation/admin-guide/mm/hugetlbpage.rst for more
18details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB are
19currently supported. Since the base page size on x86 is 4KB, a 2MB HugeTLB page
20consists of 512 base pages and a 1GB HugeTLB page consists of 4096 base pages.
21For each base page, there is a corresponding ``struct page``.
22
23Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to
24contain unique information about a HugeTLB page. ``__NR_USED_SUBPAGE`` provides
25this upper limit. The only 'useful' information in the remaining ``struct page``
26is the compound_head field, and this field is the same for all tail pages.
27
28By removing redundant ``struct page`` for HugeTLB pages, memory can be returned
29to the buddy allocator for other uses.
30
31Different architectures support different HugeTLB pages. For example, the
32following table is the HugeTLB page size supported by x86 and arm64
33architectures. Because arm64 supports 4k, 16k, and 64k base pages and
34supports contiguous entries, so it supports many kinds of sizes of HugeTLB
35page.
36
37+--------------+-----------+-----------------------------------------------+
38| Architecture | Page Size |                HugeTLB Page Size              |
39+--------------+-----------+-----------+-----------+-----------+-----------+
40|    x86-64    |    4KB    |    2MB    |    1GB    |           |           |
41+--------------+-----------+-----------+-----------+-----------+-----------+
42|              |    4KB    |   64KB    |    2MB    |    32MB   |    1GB    |
43|              +-----------+-----------+-----------+-----------+-----------+
44|    arm64     |   16KB    |    2MB    |   32MB    |     1GB   |           |
45|              +-----------+-----------+-----------+-----------+-----------+
46|              |   64KB    |    2MB    |  512MB    |    16GB   |           |
47+--------------+-----------+-----------+-----------+-----------+-----------+
48
49When the system boot up, every HugeTLB page has more than one ``struct page``
50structs which size is (unit: pages)::
51
52   struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
53
54Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
55of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
56relationship::
57
58   HugeTLB_Size = n * PAGE_SIZE
59
60Then::
61
62   struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
63               = n * sizeof(struct page) / PAGE_SIZE
64
65We can use huge mapping at the pud/pmd level for the HugeTLB page.
66
67For the HugeTLB page of the pmd level mapping, then::
68
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)
72               = 64 / 8
73               = 8 (pages)
74
75Where n is how many pte entries which one page can contains. So the value of
76n is (PAGE_SIZE / sizeof(pte_t)).
77
78This optimization only supports 64-bit system, so the value of sizeof(pte_t)
79is 8. And this optimization also applicable only when the size of ``struct page``
80is a power of two. In most cases, the size of ``struct page`` is 64 bytes (e.g.
81x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
82size of ``struct page`` structs of it is 8 page frames which size depends on the
83size of the base page.
84
85For the HugeTLB page of the pud level mapping, then::
86
87   struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
88               = PAGE_SIZE / 8 * 8 (pages)
89               = PAGE_SIZE (pages)
90
91Where the struct_size(pmd) is the size of the ``struct page`` structs of a
92HugeTLB page of the pmd level mapping.
93
94E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
95HugeTLB page consists in 4096.
96
97Next, we take the pmd level mapping of the HugeTLB page as an example to
98show the internal implementation of this optimization. There are 8 pages
99``struct page`` structs associated with a HugeTLB page which is pmd mapped.
100
101Here is how things look before optimization::
102
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 |           |                     +-----------+                +-----------+
121 |           |
122 |           |
123 |           |
124 +-----------+
125
126The value of page->compound_head is the same for all tail pages. The first
127page 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
129pages of ``struct page`` (page 1 to page 7) is to point to page->compound_head.
130Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page``
131will be used for each HugeTLB page. This will allow us to free the remaining
1327 pages to the buddy allocator.
133
134Here is how things look after remapping::
135
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 |           |                     +-----------+
154 |           |
155 |           |
156 |           |
157 +-----------+
158
159When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
160vmemmap pages and restore the previous mapping relationship.
161
162For the HugeTLB page of the pud level mapping. It is similar to the former.
163We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
164
165Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
166(e.g. aarch64) provides a contiguous bit in the translation table entries
167that hints to the MMU to indicate that it is one of a contiguous set of
168entries that can be cached in a single TLB entry.
169
170The 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
172size of the ``struct page`` structs is greater than **1** page.
173
174Notice: The head vmemmap page is not freed to the buddy allocator and all
175tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
176more than one ``struct page`` struct with ``PG_head`` (e.g. 8 per 2 MB HugeTLB
177page) associated with each HugeTLB page. The ``compound_head()`` can handle
178this correctly. There is only **one** head ``struct page``, the tail
179``struct page`` with ``PG_head`` are fake head ``struct page``.  We need an
180approach to distinguish between those two different types of ``struct page`` so
181that ``compound_head()`` can return the real head ``struct page`` when the
182parameter is the tail ``struct page`` but with ``PG_head``. The following code
183snippet describes how to distinguish between real and fake head ``struct page``.
184
185.. code-block:: c
186
187	if (test_bit(PG_head, &page->flags)) {
188		unsigned long head = READ_ONCE(page[1].compound_head);
189
190		if (head & 1) {
191			if (head == (unsigned long)page + 1)
192				/* head struct page */
193			else
194				/* tail struct page */
195		} else {
196			/* head struct page */
197		}
198	}
199
200We can safely access the field of the **page[1]** with ``PG_head`` because the
201page is a compound page composed with at least two contiguous pages.
202The implementation refers to ``page_fixed_fake_head()``.
203
204Device DAX
205==========
206
207The device-dax interface uses the same tail deduplication technique explained
208in the previous chapter, except when used with the vmemmap in
209the device (altmap).
210
211The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64),
212PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64).
213
214The differences with HugeTLB are relatively minor.
215
216It only use 3 ``struct page`` for storing all information as opposed
217to 4 on HugeTLB pages.
218
219There's no remapping of vmemmap given that device-dax memory is not part of
220System RAM ranges initialized at boot. Thus the tail page deduplication
221happens at a later stage when we populate the sections. HugeTLB reuses the
222the head vmemmap page representing, whereas device-dax reuses the tail
223vmemmap page. This results in only half of the savings compared to HugeTLB.
224
225Deduplicated tail pages are not mapped read-only.
226
227Here's how things look like on device-dax after the sections are populated::
228
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 |           |                     +-----------+
246 |           |
247 |           |
248 |           |
249 +-----------+
250