1.. SPDX-License-Identifier: GPL-2.0
2
3==========
4Device DAX
5==========
6
7The device-dax interface uses the tail deduplication technique explained in
8Documentation/mm/vmemmap_dedup.rst
9
10On powerpc, vmemmap deduplication is only used with radix MMU translation. Also
11with a 64K page size, only the devdax namespace with 1G alignment uses vmemmap
12deduplication.
13
14With 2M PMD level mapping, we require 32 struct pages and a single 64K vmemmap
15page can contain 1024 struct pages (64K/sizeof(struct page)). Hence there is no
16vmemmap deduplication possible.
17
18With 1G PUD level mapping, we require 16384 struct pages and a single 64K
19vmemmap page can contain 1024 struct pages (64K/sizeof(struct page)). Hence we
20require 16 64K pages in vmemmap to map the struct page for 1G PUD level mapping.
21
22Here's how things look like on device-dax after the sections are populated::
23 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
24 |           |                     |     0     | -------------> |     0     |
25 |           |                     +-----------+                +-----------+
26 |           |                     |     1     | -------------> |     1     |
27 |           |                     +-----------+                +-----------+
28 |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
29 |           |                     +-----------+                   | | | | |
30 |           |                     |     3     | ------------------+ | | | |
31 |           |                     +-----------+                     | | | |
32 |           |                     |     4     | --------------------+ | | |
33 |    PUD    |                     +-----------+                       | | |
34 |   level   |                     |     .     | ----------------------+ | |
35 |  mapping  |                     +-----------+                         | |
36 |           |                     |     .     | ------------------------+ |
37 |           |                     +-----------+                           |
38 |           |                     |     15    | --------------------------+
39 |           |                     +-----------+
40 |           |
41 |           |
42 |           |
43 +-----------+
44
45
46With 4K page size, 2M PMD level mapping requires 512 struct pages and a single
474K vmemmap page contains 64 struct pages(4K/sizeof(struct page)). Hence we
48require 8 4K pages in vmemmap to map the struct page for 2M pmd level mapping.
49
50Here's how things look like on device-dax after the sections are populated::
51
52 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
53 |           |                     |     0     | -------------> |     0     |
54 |           |                     +-----------+                +-----------+
55 |           |                     |     1     | -------------> |     1     |
56 |           |                     +-----------+                +-----------+
57 |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
58 |           |                     +-----------+                   | | | | |
59 |           |                     |     3     | ------------------+ | | | |
60 |           |                     +-----------+                     | | | |
61 |           |                     |     4     | --------------------+ | | |
62 |    PMD    |                     +-----------+                       | | |
63 |   level   |                     |     5     | ----------------------+ | |
64 |  mapping  |                     +-----------+                         | |
65 |           |                     |     6     | ------------------------+ |
66 |           |                     +-----------+                           |
67 |           |                     |     7     | --------------------------+
68 |           |                     +-----------+
69 |           |
70 |           |
71 |           |
72 +-----------+
73
74With 1G PUD level mapping, we require 262144 struct pages and a single 4K
75vmemmap page can contain 64 struct pages (4K/sizeof(struct page)). Hence we
76require 4096 4K pages in vmemmap to map the struct pages for 1G PUD level
77mapping.
78
79Here's how things look like on device-dax after the sections are populated::
80
81 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
82 |           |                     |     0     | -------------> |     0     |
83 |           |                     +-----------+                +-----------+
84 |           |                     |     1     | -------------> |     1     |
85 |           |                     +-----------+                +-----------+
86 |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
87 |           |                     +-----------+                   | | | | |
88 |           |                     |     3     | ------------------+ | | | |
89 |           |                     +-----------+                     | | | |
90 |           |                     |     4     | --------------------+ | | |
91 |    PUD    |                     +-----------+                       | | |
92 |   level   |                     |     .     | ----------------------+ | |
93 |  mapping  |                     +-----------+                         | |
94 |           |                     |     .     | ------------------------+ |
95 |           |                     +-----------+                           |
96 |           |                     |   4095    | --------------------------+
97 |           |                     +-----------+
98 |           |
99 |           |
100 |           |
101 +-----------+
102