Re: [RFC PATCH 2/2] userfaultfd: remove anon-vma lock for moving folios in MOVE ioctl

[David Hildenbrand <david@redhat.com>]

On 08.09.25 06:49, Lokesh Gidra wrote:
> Since rmap_walk() is now always called on locked anon folios, we don't
> have to serialize on anon_vma lock when updating folio->mapping.

I would write that as

"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 one a src VMA to a dst VMA.

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.

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 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. 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."

Something like that.

> 
> This helps avoid contention on src anon_vma when multiple threads are
> simultaneously moving distinct pages from the same src vma.
> 
> 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>
> ---



-- 
Cheers

David / dhildenb