1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==========
4 Device DAX
5 ==========
6
7 The device-dax interface uses the tail dedupli
8 Documentation/mm/vmemmap_dedup.rst
9
10 On powerpc, vmemmap deduplication is only used
11 with a 64K page size, only the devdax namespac
12 deduplication.
13
14 With 2M PMD level mapping, we require 32 struc
15 page can contain 1024 struct pages (64K/sizeof
16 vmemmap deduplication possible.
17
18 With 1G PUD level mapping, we require 16384 st
19 vmemmap page can contain 1024 struct pages (64
20 require 16 64K pages in vmemmap to map the str
21
22 Here's how things look like on device-dax afte
23 +-----------+ ---virt_to_page---> +----------
24 | | | 0
25 | | +----------
26 | | | 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
46 With 4K page size, 2M PMD level mapping requir
47 4K vmemmap page contains 64 struct pages(4K/si
48 require 8 4K pages in vmemmap to map the struc
49
50 Here's how things look like on device-dax afte
51
52 +-----------+ ---virt_to_page---> +----------
53 | | | 0
54 | | +----------
55 | | | 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
74 With 1G PUD level mapping, we require 262144 s
75 vmemmap page can contain 64 struct pages (4K/s
76 require 4096 4K pages in vmemmap to map the st
77 mapping.
78
79 Here's how things look like on device-dax afte
80
81 +-----------+ ---virt_to_page---> +----------
82 | | | 0
83 | | +----------
84 | | | 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 +-----------+
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