[PATCHv7.1 17/18] hugetlb: Update vmemmap_dedup.rst

Kiryl Shutsemau (Meta)
2026-03-02T10:56:36Z

Update the documentation regarding vmemmap optimization for hugetlb to
reflect the changes in how the kernel maps the tail pages.

Fake heads no longer exist. Remove their description.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
---
Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++-----------------
1 file changed, 26 insertions(+), 34 deletions(-)

v7.1:
- Add missing period (Randy);
- s/per-node/per-zone/ (Muchun);

diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
index 1863d88d2dcb..9fa8642ded48 100644
--- a/Documentation/mm/vmemmap_dedup.rst
+++ b/Documentation/mm/vmemmap_dedup.rst
@@ -124,33 +124,35 @@ Here is how things look before optimization::
| |
+-----------+

-The value of page->compound_info is the same for all tail pages. The first
-page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4
-``struct page`` necessary to describe the HugeTLB. The only use of the remaining
-pages of ``struct page`` (page 1 to page 7) is to point to page->compound_info.
-Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page``
-will be used for each HugeTLB page. This will allow us to free the remaining
-7 pages to the buddy allocator.
+The first page of ``struct page`` (page 0) associated with the HugeTLB page
+contains the 4 ``struct page`` necessary to describe the HugeTLB. The remaining
+pages of ``struct page`` (page 1 to page 7) are tail pages.
+
+The optimization is only applied when the size of the struct page is a power
+of 2. In this case, all tail pages of the same order are identical. See
+compound_head(). This allows us to remap the tail pages of the vmemmap to a
+shared, read-only page. The head page is also remapped to a new page. This
+allows the original vmemmap pages to be freed.

Here is how things look after remapping::

- HugeTLB struct pages(8 pages) page frame(8 pages)
- +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
- | | | 0 | -------------> | 0 |
- | | +-----------+ +-----------+
- | | | 1 | ---------------^ ^ ^ ^ ^ ^ ^
- | | +-----------+ | | | | | |
- | | | 2 | -----------------+ | | | | |
- | | +-----------+ | | | | |
- | | | 3 | -------------------+ | | | |
- | | +-----------+ | | | |
- | | | 4 | ---------------------+ | | |
- | PMD | +-----------+ | | |
- | level | | 5 | -----------------------+ | |
- | mapping | +-----------+ | |
- | | | 6 | -------------------------+ |
- | | +-----------+ |
- | | | 7 | ---------------------------+
+ HugeTLB struct pages(8 pages) page frame (new)
+ +-----------+ ---virt_to_page---> +-----------+ mapping to +----------------+
+ | | | 0 | -------------> | 0 |
+ | | +-----------+ +----------------+
+ | | | 1 | ------┐
+ | | +-----------+ |
+ | | | 2 | ------┼ +----------------------------+
+ | | +-----------+ | | A single, per-zone page |
+ | | | 3 | ------┼------> | frame shared among all |
+ | | +-----------+ | | hugepages of the same size |
+ | | | 4 | ------┼ +----------------------------+
+ | | +-----------+ |
+ | | | 5 | ------┼
+ | PMD | +-----------+ |
+ | level | | 6 | ------┼
+ | mapping | +-----------+ |
+ | | | 7 | ------┘
| | +-----------+
| |
| |
@@ -172,16 +174,6 @@ The contiguous bit is used to increase the mapping size at the pmd and pte
(last) level. So this type of HugeTLB page can be optimized only when its
size of the ``struct page`` structs is greater than **1** page.

-Notice: The head vmemmap page is not freed to the buddy allocator and all
-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. There is only **one** head ``struct page``, the tail
-``struct page`` with ``PG_head`` are fake head ``struct page``. We need an
-approach to distinguish between those two different types of ``struct page`` so
-that ``compound_head()`` can return the real head ``struct page`` when the
-parameter is the tail ``struct page`` but with ``PG_head``.
-
Device DAX
==========

--
2.51.2