Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in move_pages()

[David Hildenbrand (Arm)]

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