[PATCH 12/12] btrfs: add correction to handle -1 edge case in async discard

[Dennis Zhou <dennis@kernel.org>]

From Dave's testing, it's possible to drive a file system to have -1
discardable_extents and a corresponding negative discardable_bytes. As
btrfs_discard_calc_delay() is the only user of discardable_extents, we
can correct here for any negative discardable_extents/discardable_bytes.

Reported-by: David Sterba <dsterba@suse.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
 fs/btrfs/discard.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index d5a89e3755ed..d2c7851e31de 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -518,14 +518,32 @@ void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl)
 {
 	s32 discardable_extents =
 		atomic_read(&discard_ctl->discardable_extents);
+	s64 discardable_bytes = atomic64_read(&discard_ctl->discardable_bytes);
 	unsigned iops_limit;
 	unsigned long delay, lower_limit = BTRFS_DISCARD_MIN_DELAY_MSEC;
 
-	if (!discardable_extents)
-		return;
-
 	spin_lock(&discard_ctl->lock);
 
+	/*
+	 * The following is to fix a potential -1 discrepenancy that I'm not
+	 * sure how to reproduce.  But given that this is the only place that
+	 * utilizes these numbers and this is only called by from
+	 * btrfs_finish_extent_commit() which is synchronized, we can correct
+	 * here.
+	 */
+	if (discardable_extents < 0)
+		atomic_add(-discardable_extents,
+			   &discard_ctl->discardable_extents);
+
+	if (discardable_bytes < 0)
+		atomic64_add(-discardable_bytes,
+			     &discard_ctl->discardable_bytes);
+
+	if (discardable_extents <= 0) {
+		spin_unlock(&discard_ctl->lock);
+		return;
+	}
+
 	iops_limit = READ_ONCE(discard_ctl->iops_limit);
 	if (iops_limit)
 		lower_limit = max_t(unsigned long, lower_limit,
-- 
2.17.1