Re: [PATCH v2 2/2] mm/userfaultfd: don't lock anon_vma when performing UFFDIO_MOVE

[David Hildenbrand <david@redhat.com>]

On 23.09.25 09:10, Lokesh Gidra wrote:
> Now that rmap_walk() is guaranteed to be called with the folio lock
> held, we can stop serializing on the src VMA anon_vma lock when moving
> an exclusive folio from a src VMA to a dst VMA in UFFDIO_MOVE ioctl.
> 
> When moving a folio, we modify folio->mapping through
> folio_move_anon_rmap() and adjust folio->index accordingly. Doing that
> while we could have concurrent RMAP walks would be dangerous. Therefore,
> to avoid that, we had to acquire anon_vma of src VMA in write-mode. That
> meant that when multiple threads called UFFDIO_MOVE concurrently on
> distinct pages of the same src VMA, they would serialize on it, hurting
> scalability.
> 
> In addition to avoiding the scalability bottleneck, this patch also
> simplifies the complicated lock dance that UFFDIO_MOVE has to go through
> between RCU, folio-lock, ptl, and anon_vma.
> 
> folio_move_anon_rmap() already enforces that the folio is locked. So
> when we have the folio locked we can no longer race with concurrent
> rmap_walk() as used by folio_referenced() and others who call it on
> unlocked non-KSM anon folios, and therefore the anon_vma lock is no
> longer required.
> 
> Note that this handling is now the same as for other
> folio_move_anon_rmap() users that also do not hold the anon_vma lock --
> namely COW reuse handling (do_wp_page()->wp_can_reuse_anon_folio(),
> do_huge_pmd_wp_page(), and hugetlb_wp()). These users never required the
> anon_vma lock as they are only moving the anon VMA closer to the anon_vma
> leaf of the VMA, for example, from an anon_vma root to a leaf of that root.
> rmap walks were always able to tolerate that scenario.
> 
> CC: David Hildenbrand <david@redhat.com>
> CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> CC: Peter Xu <peterx@redhat.com>
> CC: Suren Baghdasaryan <surenb@google.com>
> CC: Barry Song <baohua@kernel.org>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb