Re: [RFC] Unconditionally lock folios when calling rmap_walk()

[Lokesh Gidra <lokeshgidra@google.com>]

Adding linux-mm mailing list. Mistakenly used the wrong email address.

On Wed, Aug 20, 2025 at 9:23 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> Hi all,
>
> Currently, some callers of rmap_walk() conditionally avoid try-locking
> non-ksm anon folios. This necessitates serialization through anon_vma
> write-lock when folio->mapping and/or folio->index (fields involved in
> rmap_walk()) are to be updated. This hurts scalability due to coarse
> granularity of the lock. For instance, when multiple threads invoke
> userfaultfd’s MOVE ioctl simultaneously to move distinct pages from
> the same src VMA, they all contend for the corresponding anon_vma’s
> lock. Field traces for arm64 android devices reveal over 30ms of
> uninterruptible sleep in the main UI thread, leading to janky user
> interactions.
>
> Among all rmap_walk() callers that don’t lock anon folios,
> folio_referenced() is the most critical (others are
> page_idle_clear_pte_refs(), damon_folio_young(), and
> damon_folio_mkold()). The relevant code in folio_referenced() is:
>
> if (!is_locked && (!folio_test_anon(folio) || folio_test_ksm(folio))) {
>         we_locked = folio_trylock(folio);
>         if (!we_locked)
>                 return 1;
> }
>
> It’s unclear why locking anon_vma (when updating folio->mapping) is
> beneficial over locking the folio here. It’s in the reclaim path, so
> should not be a critical path that necessitates some special
> treatment, unless I’m missing something.
>
> Therefore, I propose simplifying the locking mechanism by
> unconditionally try-locking the folio in such cases. This helps avoid
> locking anon_vma when updating folio->mapping, which, for instance,
> will help eliminate the uninterruptible sleep observed in the field
> traces mentioned earlier. Furthermore, it enables us to simplify the
> code in folio_lock_anon_vma_read() by removing the re-check to ensure
> that the field hasn’t changed under us.
>
> Thanks,
> Lokesh