Re: [PATCH] mm: remove an avoidable load of page refcount in page_ref_add_unless

[Mateusz Guzik]

On Mon, Dec 9, 2024 at 10:28 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 07.12.24 09:29, Mateusz Guzik wrote:
> > Explicitly pre-checking the count adds nothing as atomic_add_unless
> > starts with doing the same thing. iow no functional changes.
>
> I recall that we added that check because with the hugetlb vmemmap
> optimization, some of the tail pages we don't ever expect to be modified
>   (because they are fake-duplicated) might be mapped R/O.
>
> If the arch implementation of atomic_add_unless() would trigger an
> unconditional write fault, we'd be in trouble. That would likely only be
> the case if the arch provides a dedicate instruction.
>
> atomic_add_unless()->raw_atomic_add_unless()
>
> Nobody currently defines arch_atomic_add_unless().
>
> raw_atomic_fetch_add_unless()->arch_atomic_fetch_add_unless() is defined
> on some architectures.
>
> I scanned some of the inline-asm, and I think most of them perform a
> check first.
>

Huh.

Some arch triggering a write fault despite not changing the value is
not something I thought about. Sounds pretty broken to me if any arch
was to do it, but then stranger things did happen.

However, if this is seen as a real concern, then I think the best way
forward is to drop the patch (and maybe instead add a comment what's
up with the extra load).
-- 
Mateusz Guzik <mjguzik gmail.com>