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Diffstat (limited to 'Documentation/vm/vmemmap_dedup.rst')
| -rw-r--r-- | Documentation/vm/vmemmap_dedup.rst | 56 |
1 files changed, 53 insertions, 3 deletions
diff --git a/Documentation/vm/vmemmap_dedup.rst b/Documentation/vm/vmemmap_dedup.rst index 485ccf4f7b10..c9c495f62d12 100644 --- a/Documentation/vm/vmemmap_dedup.rst +++ b/Documentation/vm/vmemmap_dedup.rst @@ -1,8 +1,11 @@ .. SPDX-License-Identifier: GPL-2.0 -================================== -Free some vmemmap pages of HugeTLB -================================== +========================================= +A vmemmap diet for HugeTLB and Device DAX +========================================= + +HugeTLB +======= The struct page structures (page structs) are used to describe a physical page frame. By default, there is a one-to-one mapping from a page frame to @@ -171,3 +174,50 @@ tail vmemmap pages are mapped to the head vmemmap page frame. So we can see more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page) associated with each HugeTLB page. The compound_head() can handle this correctly (more details refer to the comment above compound_head()). + +Device DAX +========== + +The device-dax interface uses the same tail deduplication technique explained +in the previous chapter, except when used with the vmemmap in +the device (altmap). + +The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64), +PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64). + +The differences with HugeTLB are relatively minor. + +It only use 3 page structs for storing all information as opposed +to 4 on HugeTLB pages. + +There's no remapping of vmemmap given that device-dax memory is not part of +System RAM ranges initialized at boot. Thus the tail page deduplication +happens at a later stage when we populate the sections. HugeTLB reuses the +the head vmemmap page representing, whereas device-dax reuses the tail +vmemmap page. This results in only half of the savings compared to HugeTLB. + +Deduplicated tail pages are not mapped read-only. + +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 | --------------------------+ + | | +-----------+ + | | + | | + | | + +-----------+ |