Re: [PATCHv6 16/17] hugetlb: Update vmemmap_dedup.rst

[David Hildenbrand (Arm)]

On 2/2/26 16:56, Kiryl Shutsemau wrote:
> 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@xxxxxxxxxx>
> Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
> ---
>   Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++-----------------
>   1 file changed, 26 insertions(+), 34 deletions(-)
>
> diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
> index 1863d88d2dcb..fca9d0ce282a 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

You could highlight that we allocate a new head page like "page frame 
(new)".

Wasn't aware of that detail before reading your change above.

Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

--
Cheers,

David