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

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Message-ID: <Zg3u5Sh9EbbYPhaI@casper.infradead.org>
Date: Thu, 4 Apr 2024 01:05:57 +0100
From: Matthew Wilcox <willy@...radead.org>
To: peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Huacai Chen <chenhuacai@...nel.org>,
	David Hildenbrand <david@...hat.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.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 Tue, Apr 02, 2024 at 09:32:47PM -0400, peterx@...hat.com wrote:
> +++ 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);

I think this should be written as:

	/*
	 * HugeTLB stores this information on the head page; THP keeps
	 * it per page
	 */
	if (PageHuge(page))
		page = compound_head(page);

>  	return test_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags);
>  }