From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Matthew Wilcox <willy@infradead.org>,
Aswin Kumar <aswinkumar3301@gmail.com>
Cc: linux-mm@kvack.org, andrew.morton@linux-foundation.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in move_pages()
Date: Thu, 12 Feb 2026 13:47:34 +0100 [thread overview]
Message-ID: <21a9b331-58ec-4c04-a09e-545ac31f2eca@kernel.org> (raw)
In-Reply-To: <aYNemlPABKnQkf-x@casper.infradead.org>
On 2/4/26 15:58, Matthew Wilcox wrote:
> On Wed, Feb 04, 2026 at 02:23:50PM +0000, Aswin Kumar wrote:
>> All other userfaultfd paths use unlikely() for the mmap_changing check,
>
> No they don't?
>
> $ git grep mmap_changing mm
> mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
> mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
> mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
> mm/userfaultfd.c: if (likely(atomic_read(&ctx->mmap_changing)))
>
>> This is a performance-only fix - the logic is correct but the branch
>> prediction annotation is wrong, potentially causing a minor performance
>> penalty on the fast path.
>
> Can you measure it? Bet you can't.
>
> Honestly, I'd just remove the likely() annotation, not change it to
> unlikely().
Agreed.
--
Cheers,
David
prev parent reply other threads:[~2026-02-12 12:47 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-04 14:23 Aswin Kumar
2026-02-04 14:58 ` Matthew Wilcox
2026-02-12 12:47 ` David Hildenbrand (Arm) [this message]
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