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Message-ID: <98887861-b9d4-40a1-8c8a-6a417ba475f7@linux.dev> Date: Mon, 22 Dec 2025 14:20:29 +0800 From: Muchun Song <muchun.song@...ux.dev> To: Kiryl Shutsemau <kas@...nel.org> Cc: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>, Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>, kernel-team@...a.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...nel.org>, Matthew Wilcox <willy@...radead.org>, Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com> Subject: Re: [PATCHv2 14/14] hugetlb: Update vmemmap_dedup.rst On 2025/12/18 23:09, 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@...nel.org> > --- > 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..a0c4c79d6922 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. Replacing the head page is nice-to-have, so I think the details of it should not mentioned here. > > 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 > + +-----------+ ---virt_to_page---> +-----------+ mapping to +----------------+ > + | | | 0 | -------------> | 0 | > + | | +-----------+ +----------------+ > + | | | 1 | ------┐ > + | | +-----------+ | > + | | | 2 | ------┼ +----------------+ > + | | +-----------+ | | vmemmap_tail | > + | | | 3 | ------┼------> | shared for the | > + | | +-----------+ | | struct hstate | I suggest using the following wording (since struct hstate and vmemmap_tail are somewhat code-level implementation details). A single, per-node page frame shared among all hugepages of the same size Thanks. > + | | | 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 > ========== >
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