linux-kernel - Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in move

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Message-ID: <aYNemlPABKnQkf-x@casper.infradead.org>
Date: Wed, 4 Feb 2026 14:58:34 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Aswin Kumar <aswinkumar3301@...il.com>
Cc: linux-mm@...ck.org, andrew.morton@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in
 move_pages()

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().