From 8ccb6ded6a47b59b4907ed4f8c293cf8122de889 Mon Sep 17 00:00:00 2001 From: Josef Bacik Date: Sat, 31 Oct 2026 10:39:36 +0100 Subject: [PATCH] mm/filemap: don't lose a folio lock wakeup to a signal An exclusive waiter in folio_wait_bit_common() can leave the wait loop because of a fatal signal, then be woken by folio_unlock() before finish_wait(). If the waker did not hand over the lock, the waiter returns -EINTR without trying to acquire it. folio_wake_bit() stops after waking the first exclusive waiter, so this loses the wakeup and can leave other waiters asleep indefinitely on an unlocked folio. After finish_wait(), let a waiter woken without a lock handoff try once more to acquire the lock. If successful, return success so the caller's eventual unlock wakes the next waiter. Otherwise, the current lock owner will do so. Fix generated by an LLM and sent by Josef Bacik in private. Fixes: 5ef64cc8987a ("mm: allow a controlled amount of unfairness in the page lock") Cc: stable@vger.kernel.org Signed-off-by: Josef Bacik [ david: clarify and simplify comment + description ] Reviewed-by: Rik van Riel Acked-by: Pedro Falcato Tested-by: Muhammad Usama Anjum Link: https://patch.msgid.link/20261031-fixes_core-v3-7-d6437171ecc3@kernel.org Signed-off-by: David Hildenbrand (Arm) --- mm/filemap.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 906b013c11eb..6b5e1d866a9e 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1373,10 +1373,22 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr, * return value based on that state without races. * * Also note that WQ_FLAG_WOKEN is sufficient for a non-exclusive - * waiter, but an exclusive one requires WQ_FLAG_DONE. + * waiter, but an exclusive one must also acquire the bit. */ - if (behavior == EXCLUSIVE) - return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR; + if (behavior == EXCLUSIVE) { + if (wait->flags & WQ_FLAG_DONE) + return 0; + /* + * We might have just been woken up and the waker will not + * wake up anybody else. If we ignore that now, the remaining + * waiters may not get woken up: take the bit now if it is + * still free. + */ + if ((wait->flags & WQ_FLAG_WOKEN) && + !test_and_set_bit(bit_nr, folio_flags(folio, 0))) + return 0; + return -EINTR; + } return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR; }