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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* 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?

>  			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
>