tjh.dev
Linux kernel mirror (for testing)
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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 +-----------+