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

[Lokesh Gidra <lokeshgidra@google.com>]

On Fri, Aug 22, 2025 at 10:29 AM 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 elsewhere 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 exclusively (when updating
> folio->mapping, like in uffd MOVE) is beneficial over walking rmap
> with folio locked. 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 ensuring the
> folio is locked before calling rmap_walk(). 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.
Hi Harry,

Your comment [1] in the other thread was quite useful and also needed
to be responded to. So bringing it here for continuing discussion.

It seems from your comment that you misunderstood my proposal. I am
not suggesting replacing anon_vma lock with folio lock during rmap
walk. Clearly, it is essential for all the reasons that you
enumerated. My proposal is to lock anon folios during rmap_walk(),
like file and KSM folios.

This helps in improving scalability (and also simplifying code in
folio_lock_anon_vma_read()) as then we can serialize on folio lock
instead of anon_vma lock when moving the folio to a different root
anon_vma in folio_move_anon_rmap() [2].

[1] https://lore.kernel.org/all/aKhIL3OguViS9myH@hyeyoo/
[2] https://lore.kernel.org/all/e5d41fbe-a91b-9491-7b93-733f67e75a54@redhat.com/
>
> Thanks,
> Lokesh