btrfs: don't block if we can't acquire the reclaim lock

If we can't acquire the reclaim_bgs_lock on block group reclaim, we
block until it is free. This can potentially stall for a long time.

While reclaim of block groups is necessary for a good user experience on
a zoned file system, there still is no need to block as it is best
effort only, just like when we're deleting unused block groups.

CC: stable@vger.kernel.org # 5.13
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index c557327..9e7d9d0 100644 (file)
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1498,7 +1498,15 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
        if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE))
                return;
 
-       mutex_lock(&fs_info->reclaim_bgs_lock);
+       /*
+        * Long running balances can keep us blocked here for eternity, so
+        * simply skip reclaim if we're unable to get the mutex.
+        */
+       if (!mutex_trylock(&fs_info->reclaim_bgs_lock)) {
+               btrfs_exclop_finish(fs_info);
+               return;
+       }
+
        spin_lock(&fs_info->unused_bgs_lock);
        while (!list_empty(&fs_info->reclaim_bgs)) {
                u64 zone_unusable;