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

Suren Baghdasaryan
2025-02-25T23:02:06Z

On Tue, Feb 25, 2025 at 2:48 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Feb 25, 2025 at 02:21:39PM -0800, Suren Baghdasaryan wrote:
> > On Tue, Feb 25, 2025 at 2:12 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Tue, Feb 25, 2025 at 1:32 PM Peter Xu <peterx@redhat.com> wrote:
> > > >
> > > > On Tue, Feb 25, 2025 at 12:46:13PM -0800, Suren Baghdasaryan wrote:
> > > > > 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>
> > > >
> > > > Reviewed-by: Peter Xu <peterx@redhat.com>
> > > >
> > > > One question irrelevant of this change below..
> > > >
> > > > > ---
> > > > > 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;
> > > > > + }
> > > > > +
> > > > > 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);
> > > >
> > > > .. just notice this. Are these problematic? I mean, orig_*_pte are stack
> > > > variables, afaict. I'd expect these things blow on HIGHPTE..
> > >
> > > Ugh! Yes, I think so. From a quick look, move_pages_pte() is the only
> > > place we have this issue and I don't see a reason for copying src_pte
> > > and dst_pte values. I'll spend some more time trying to understand if
> > > we really need these local copies.
> >
> > Ah, we copy the values to later check if PTEs changed from under us.
> > But I see no reason we need to use orig_{src|dst}_pte instead of
> > {src|dst}_pte when doing pte_unmap(). I think we can safely replace
>
> That looks like something we just overlooked before, meanwhile it's
> undetectable on !HIGHPTE anyway.. in which case the addr ignored, and that
> turns always into an rcu unlock.
>
> > them with the original ones. WDYT?
>
> Agreed. Maybe not "the original ones" if we're looking for words to put
> into the commit message: it could be "we should kunmap() whatever we
> kmap()ed before", or something better.

Sounds good to me. I'll give others one day to provide their input and
will post a fix.
Thanks!

>
> Thanks,
>
> --
> Peter Xu
>