linux-kernel - Re: [PATCHv6 12/17] mm: Drop fake head checks

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Message-ID: <f417576d-ab79-44f5-99a1-3f672b490bf2@kernel.org>
Date: Fri, 6 Feb 2026 10:41:45 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Matthew Wilcox <willy@...radead.org>,
 Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>
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>,
 Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, kernel-team@...a.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-riscv@...ts.infradead.org
Subject: Re: [PATCHv6 12/17] mm: Drop fake head checks

On 2/2/26 16:56, Kiryl Shutsemau wrote:
> With fake head pages eliminated in the previous commit, remove the
> supporting infrastructure:
> 
>    - page_fixed_fake_head(): no longer needed to detect fake heads;
>    - page_is_fake_head(): no longer needed;
>    - page_count_writable(): no longer needed for RCU protection;
>    - RCU read_lock in page_ref_add_unless(): no longer needed;
> 
> This substantially simplifies compound_head() and page_ref_add_unless(),
> removing both branches and RCU overhead from these hot paths.

Can you say a bit words more why RCU was required and is now no longer 
needed?

IIRC, it's because we now no longer reuse the real head page (page 0) 
for a tail, and there could have been a race where we could have 
attempted to write to that page0 while already mapped (r/o) to page1.

Also good to mention that the corresponding RCU sync will be removed 
separately in a following commit.

Nothing jumped at me and it's a great simplification for core-mm.

Acked-by: David Hildenbrand (Arm) <david@...nel.org>

-- 
Cheers,

David