Re: [PATCH 1/1] userfaultfd: do not block on locking a large folio with raised refcount

Suren Baghdasaryan
2025-02-26T16:11:42Z

On Wed, Feb 26, 2025 at 6:59 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [250225 15:46]:
> > Lokesh recently raised an issue about UFFDIO_MOVE getting into a deadlock
> > state when it goes into split_folio() with raised folio refcount.
> > split_folio() expects the reference count to be exactly
> > mapcount + num_pages_in_folio + 1 (see can_split_folio()) and fails with
> > EAGAIN otherwise. If multiple processes are trying to move the same
> > large folio, they raise the refcount (all tasks succeed in that) then
> > one of them succeeds in locking the folio, while others will block in
> > folio_lock() while keeping the refcount raised. The winner of this
> > race will proceed with calling split_folio() and will fail returning
> > EAGAIN to the caller and unlocking the folio. The next competing process
> > will get the folio locked and will go through the same flow. In the
> > meantime the original winner will be retried and will block in
> > folio_lock(), getting into the queue of waiting processes only to repeat
> > the same path. All this results in a livelock.
> > An easy fix would be to avoid waiting for the folio lock while holding
> > folio refcount, similar to madvise_free_huge_pmd() where folio lock is
> > acquired before raising the folio refcount.
> > Modify move_pages_pte() to try locking the folio first and if that fails
> > and the folio is large then return EAGAIN without touching the folio
> > refcount. If the folio is single-page then split_folio() is not called,
> > so we don't have this issue.
> > Lokesh has a reproducer [1] and I verified that this change fixes the
> > issue.
> >
> > [1] https://github.com/lokeshgidra/uffd_move_ioctl_deadlock
> >
> > Reported-by: Lokesh Gidra <lokeshgidra@google.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > mm/userfaultfd.c | 17 ++++++++++++++++-
> > 1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 867898c4e30b..f17f8290c523 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -1236,6 +1236,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> > */
> > if (!src_folio) {
> > struct folio *folio;
> > + bool locked;
> >
> > /*
> > * Pin the page while holding the lock to be sure the
> > @@ -1255,12 +1256,26 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> > goto out;
> > }
> >
> > + locked = folio_trylock(folio);
> > + /*
> > + * We avoid waiting for folio lock with a raised refcount
> > + * for large folios because extra refcounts will result in
> > + * split_folio() failing later and retrying. If multiple
> > + * tasks are trying to move a large folio we can end
> > + * livelocking.
> > + */
> > + if (!locked && folio_test_large(folio)) {
> > + spin_unlock(src_ptl);
> > + err = -EAGAIN;
> > + goto out;
> > + }
> > +
>
> Reversing the locking/folio_get() is okay because of the src_ptl spin
> lock, right? It might be worth saying something about it in the
> comment?

That is correct. We take both folio lock and refcount before we drop
PTL. I'll add a comment. Thanks!

>
> > folio_get(folio);
> > src_folio = folio;
> > src_folio_pte = orig_src_pte;
> > spin_unlock(src_ptl);
> >
> > - if (!folio_trylock(src_folio)) {
> > + if (!locked) {
> > pte_unmap(&orig_src_pte);
> > pte_unmap(&orig_dst_pte);
> > src_pte = dst_pte = NULL;
> >
> > base-commit: 801d47bd96ce22acd43809bc09e004679f707c39
> > --
> > 2.48.1.658.g4767266eb4-goog
> >