On Wed, Feb 26, 2025 at 6:59 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [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@xxxxxxxxxx> > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > --- > > 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 > >
