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

Dennis Zhou
2020-01-02T21:27:07Z

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