Based on kernel version 6.11. Page generated on 2024-09-24 08:21 EST.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 | .. SPDX-License-Identifier: GPL-2.0 ========== Device DAX ========== The device-dax interface uses the tail deduplication technique explained in Documentation/mm/vmemmap_dedup.rst On powerpc, vmemmap deduplication is only used with radix MMU translation. Also with a 64K page size, only the devdax namespace with 1G alignment uses vmemmap deduplication. With 2M PMD level mapping, we require 32 struct pages and a single 64K vmemmap page can contain 1024 struct pages (64K/sizeof(struct page)). Hence there is no vmemmap deduplication possible. With 1G PUD level mapping, we require 16384 struct pages and a single 64K vmemmap page can contain 1024 struct pages (64K/sizeof(struct page)). Hence we require 16 64K pages in vmemmap to map the struct page for 1G PUD level mapping. Here's how things look like on device-dax after the sections are populated:: +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | +-----------+ +-----------+ | | | 1 | -------------> | 1 | | | +-----------+ +-----------+ | | | 2 | ----------------^ ^ ^ ^ ^ ^ | | +-----------+ | | | | | | | | 3 | ------------------+ | | | | | | +-----------+ | | | | | | | 4 | --------------------+ | | | | PUD | +-----------+ | | | | level | | . | ----------------------+ | | | mapping | +-----------+ | | | | | . | ------------------------+ | | | +-----------+ | | | | 15 | --------------------------+ | | +-----------+ | | | | | | +-----------+ With 4K page size, 2M PMD level mapping requires 512 struct pages and a single 4K vmemmap page contains 64 struct pages(4K/sizeof(struct page)). Hence we require 8 4K pages in vmemmap to map the struct page for 2M pmd level mapping. Here's how things look like on device-dax after the sections are populated:: +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | +-----------+ +-----------+ | | | 1 | -------------> | 1 | | | +-----------+ +-----------+ | | | 2 | ----------------^ ^ ^ ^ ^ ^ | | +-----------+ | | | | | | | | 3 | ------------------+ | | | | | | +-----------+ | | | | | | | 4 | --------------------+ | | | | PMD | +-----------+ | | | | level | | 5 | ----------------------+ | | | mapping | +-----------+ | | | | | 6 | ------------------------+ | | | +-----------+ | | | | 7 | --------------------------+ | | +-----------+ | | | | | | +-----------+ With 1G PUD level mapping, we require 262144 struct pages and a single 4K vmemmap page can contain 64 struct pages (4K/sizeof(struct page)). Hence we require 4096 4K pages in vmemmap to map the struct pages for 1G PUD level mapping. Here's how things look like on device-dax after the sections are populated:: +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | +-----------+ +-----------+ | | | 1 | -------------> | 1 | | | +-----------+ +-----------+ | | | 2 | ----------------^ ^ ^ ^ ^ ^ | | +-----------+ | | | | | | | | 3 | ------------------+ | | | | | | +-----------+ | | | | | | | 4 | --------------------+ | | | | PUD | +-----------+ | | | | level | | . | ----------------------+ | | | mapping | +-----------+ | | | | | . | ------------------------+ | | | +-----------+ | | | | 4095 | --------------------------+ | | +-----------+ | | | | | | +-----------+ |