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Documentation/mm: update DAX vmemmap deduplication docs

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Device DAX now uses the common per-zone shared tail page for vmemmap
deduplication.  The old documentation still described a DAX-specific
layout with a separately populated tail vmemmap page and half the HugeTLB
savings.

Update the generic and powerpc documentation to describe the shared
layout.  In the powerpc document, keep the radix and 64K-specific details,
drop the duplicated 4K PUD arithmetic, and replace the repeated device-dax
diagrams with a single parameterized PMD/PUD diagram.

Link: https://lore.kernel.org/20260930140627.57431-13-songmuchun@bytedance.com
Signed-off-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Lance Yang <lance.yang@linux.dev>
This commit is contained in:
Muchun Song 2026-09-30 22:06:27 +08:00 committed by Andrew Morton
parent 76df418d29
commit 27ffeec7b0
2 changed files with 20 additions and 100 deletions
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88
Documentation/arch/powerpc/vmemmap_dedup.rst
View File

@ -19,82 +19,28 @@ 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::
Here's how things look on device-dax after vmemmap-optimized sections are
populated. ``N`` is the number of vmemmap pages required by the DAX mapping
above::
+-----------+ ---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 | --------------------------+
| | +-----------+
Device DAX vmemmap pages (N pages) backing page frames
+-----------+ ---virt_to_page---> +-----------+ mapping to +-------------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-------------+
| | | 1 | ------+
| | +-----------+ |
| | | 2 | ------+
| | +-----------+ |
| | | . | ------+ +-------------+
| PMD/PUD | +-----------+ | | A single, |
| level | | . | ------+------> | per-zone |
| mapping | +-----------+ | | shared tail |
| | | N - 1 | ------+ | page |
| | +-----------+ +-------------+
| |
| |
| |

32
Documentation/mm/vmemmap_dedup.rst
View File

@ -1,4 +1,3 @@
.. SPDX-License-Identifier: GPL-2.0
=========================================
@ -192,32 +191,7 @@ 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.
happens at a later stage when we populate the sections.
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 | --------------------------+
| | +-----------+
| |
| |
| |
+-----------+
Deduplicated tail pages are not mapped read-only. The mapping layout is the same
as HugeTLB.