Device DAX¶
The device-dax interface uses the tail deduplication technique explained in A vmemmap diet for HugeTLB and Device DAX
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 | --------------------------+
| | +-----------+
| |
| |
| |
+-----------+