linux-kernel - Re: [PATCHv4 13/14] mm: Remove the branch from compound

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Message-ID: <576136E0-440F-4727-969D-B558C862CD19@nvidia.com>
Date: Wed, 21 Jan 2026 13:21:45 -0500
From: Zi Yan <ziy@...dia.com>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, David Hildenbrand <david@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, Usama Arif <usamaarif642@...il.com>,
 Frank van der Linden <fvdl@...gle.com>, Oscar Salvador <osalvador@...e.de>,
 Mike Rapoport <rppt@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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
Subject: Re: [PATCHv4 13/14] mm: Remove the branch from compound_head()

On 21 Jan 2026, at 11:22, Kiryl Shutsemau wrote:

> The compound_head() function is a hot path. For example, the zap path
> calls it for every leaf page table entry.
>
> Rewrite the helper function in a branchless manner to eliminate the risk
> of CPU branch misprediction.
>
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> Reviewed-by: Muchun Song <muchun.song@...ux.dev>
> ---
>  include/linux/page-flags.h | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)

LGTM. And it fixed the issue in patch 10.

Reviewed-by: Zi Yan <ziy@...dia.com>

>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f89702e101e8..95ac963b78af 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void)
>  static __always_inline unsigned long _compound_head(const struct page *page)
>  {
>  	unsigned long info = READ_ONCE(page->compound_info);
> +	unsigned long mask;
> +
> +	if (!compound_info_has_mask()) {
> +		/* Bit 0 encodes PageTail() */
> +		if (info & 1)
> +			return info - 1;
>
> -	/* Bit 0 encodes PageTail() */
> -	if (!(info & 1))
>  		return (unsigned long)page;

Here, info & 1 check now is guarded by compound_info_has_mask(), so the
issue I mentioned in Patch 10 is gone.

> -
> -	/*
> -	 * If compound_info_has_mask() is false, the rest of compound_info is
> -	 * the pointer to the head page.
> -	 */
> -	if (!compound_info_has_mask())
> -		return info - 1;
> +	}


Best Regards,
Yan, Zi