linux-kernel - Re: [PATCH 1/3] mm: Allow anon exclusive check over hugetlb tail pages

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Date: Wed, 3 Apr 2024 09:37:03 +0200
From: David Hildenbrand <david@...hat.com>
To: peterx@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: Huacai Chen <chenhuacai@...nel.org>, Jason Gunthorpe <jgg@...dia.com>,
 Nathan Chancellor <nathan@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, WANG Xuerui <kernel@...0n.name>,
 Ryan Roberts <ryan.roberts@....com>, loongarch@...ts.linux.dev
Subject: Re: [PATCH 1/3] mm: Allow anon exclusive check over hugetlb tail
 pages

On 03.04.24 03:32, peterx@...hat.com wrote:
> From: Peter Xu <peterx@...hat.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@...hat.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@...hat.com>

-- 
Cheers,

David / dhildenb