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SubjectRe: [PATCH 1/3] mm: Allow anon exclusive check over hugetlb tail pages
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On 03.04.24 03:32, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
>
> PageAnonExclusive() used to forbid tail pages for hugetlbfs, as that used
> to be called mostly in hugetlb specific paths and the head page was
> guaranteed.
>
> As we move forward towards merging hugetlb paths into generic mm, we may
> start to pass in tail hugetlb pages (when with cont-pte/cont-pmd huge
> pages) for such check. Allow it to properly fetch the head, in which case
> the anon-exclusiveness of the head will always represents the tail page.
>
> There's already a sign of it when we look at the fast-gup which already

"GUP-fast" ;)

> contain the hugetlb processing altogether: we used to have a specific
> commit 5805192c7b72 ("mm/gup: handle cont-PTE hugetlb pages correctly in
> gup_must_unshare() via GUP-fast") covering that area. Now with this more
> generic change, that can also go away.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> include/linux/page-flags.h | 8 +++++++-
> mm/internal.h | 10 ----------
> 2 files changed, 7 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 888353c209c0..225357f48a79 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -1095,7 +1095,13 @@ PAGEFLAG(Isolated, isolated, PF_ANY);
> static __always_inline int PageAnonExclusive(const struct page *page)
> {
> VM_BUG_ON_PGFLAGS(!PageAnon(page), page);
> - VM_BUG_ON_PGFLAGS(PageHuge(page) && !PageHead(page), page);
> + /*
> + * Allow the anon-exclusive check to work on hugetlb tail pages.
> + * Here hugetlb pages will always guarantee the anon-exclusiveness
> + * of the head page represents the tail pages.
> + */
> + if (PageHuge(page) && !PageHead(page))
> + page = compound_head(page);
> return test_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags);
> }
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 9512de7398d5..87f6e4fd56a5 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1259,16 +1259,6 @@ static inline bool gup_must_unshare(struct vm_area_struct *vma,
> if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> smp_rmb();
>
> - /*
> - * During GUP-fast we might not get called on the head page for a
> - * hugetlb page that is mapped using cont-PTE, because GUP-fast does
> - * not work with the abstracted hugetlb PTEs that always point at the
> - * head page. For hugetlb, PageAnonExclusive only applies on the head
> - * page (as it cannot be partially COW-shared), so lookup the head page.
> - */
> - if (unlikely(!PageHead(page) && PageHuge(page)))
> - page = compound_head(page);
> -
> /*
> * Note that PageKsm() pages cannot be exclusive, and consequently,
> * cannot get pinned.


Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb


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