btrfs: prevent pathological periodic reclaim loops

Periodic reclaim runs the risk of getting stuck in a state where it
keeps reclaiming the same block group over and over. This can happen if

1. reclaiming that block_group fails
2. reclaiming that block_group fails to move any extents into existing
   block_groups and just allocates a fresh chunk and moves everything.

Currently, 1. is a very tight loop inside the reclaim worker. That is
critical for edge triggered reclaim or else we risk forgetting about a
reclaimable group. On the other hand, with level triggered reclaim we
can break out of that loop and get it later.

With that fixed, 2. applies to both failures and "successes" with no
progress. If we have done a periodic reclaim on a space_info and nothing
has changed in that space_info, there is not much point to trying again,
so don't, until enough space gets free, which we capture with a
heuristic of needing to net free 1 chunk.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 11e0a633fd76..cb95bf7ccbbd 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1933,6 +1933,8 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 			reclaimed = 0;
 			spin_lock(&space_info->lock);
 			space_info->reclaim_errors++;
+			if (READ_ONCE(space_info->periodic_reclaim))
+				space_info->periodic_reclaim_ready = false;
 			spin_unlock(&space_info->lock);
 		}
 		spin_lock(&space_info->lock);
@@ -1941,7 +1943,7 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 		spin_unlock(&space_info->lock);
 
 next:
-		if (ret) {
+		if (ret && !READ_ONCE(space_info->periodic_reclaim)) {
 			/* Refcount held by the reclaim_bgs list after splice. */
 			spin_lock(&fs_info->unused_bgs_lock);
 			/*
@@ -3686,6 +3688,8 @@ int btrfs_update_block_group(struct btrfs_trans_handle *trans,
 		space_info->bytes_reserved -= num_bytes;
 		space_info->bytes_used += num_bytes;
 		space_info->disk_used += num_bytes * factor;
+		if (READ_ONCE(space_info->periodic_reclaim))
+			btrfs_space_info_update_reclaimable(space_info, -num_bytes);
 		spin_unlock(&cache->lock);
 		spin_unlock(&space_info->lock);
 	} else {
@@ -3695,8 +3699,10 @@ int btrfs_update_block_group(struct btrfs_trans_handle *trans,
 		btrfs_space_info_update_bytes_pinned(info, space_info, num_bytes);
 		space_info->bytes_used -= num_bytes;
 		space_info->disk_used -= num_bytes * factor;
-
-		reclaim = should_reclaim_block_group(cache, num_bytes);
+		if (READ_ONCE(space_info->periodic_reclaim))
+			btrfs_space_info_update_reclaimable(space_info, num_bytes);
+		else
+			reclaim = should_reclaim_block_group(cache, num_bytes);
 
 		spin_unlock(&cache->lock);
 		spin_unlock(&space_info->lock);