+++ /dev/null
-.. 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 | --------------------------+
- | | +-----------+
- | |
- | |
- | |
- +-----------+