[PATCH 5.15 220/356] btrfs: retry block group reclaim without infinite loop

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5.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Boris Burkov <boris@xxxxxx>

commit 4eb4e85c4f818491efc67e9373aa16b123c3f522 upstream.

If inc_block_group_ro systematically fails (e.g. due to ETXTBUSY from
swap) or btrfs_relocate_chunk systematically fails (from lack of
space), then this worker becomes an infinite loop.

At the very least, this strands the cleaner thread, but can also result
in hung tasks/RCU stalls on PREEMPT_NONE kernels and if the
reclaim_bgs_lock mutex is not contended.

I believe the best long term fix is to manage reclaim via work queue,
where we queue up a relocation on the triggering condition and re-queue
on failure. In the meantime, this is an easy fix to apply to avoid the
immediate pain.

Fixes: 7e2718099438 ("btrfs: reinsert BGs failed to reclaim")
CC: stable@xxxxxxxxxxxxxxx # 6.6+
Signed-off-by: Boris Burkov <boris@xxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/btrfs/block-group.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1493,6 +1493,7 @@ void btrfs_reclaim_bgs_work(struct work_
 		container_of(work, struct btrfs_fs_info, reclaim_bgs_work);
 	struct btrfs_block_group *bg;
 	struct btrfs_space_info *space_info;
+	LIST_HEAD(retry_list);
 
 	if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
 		return;
@@ -1583,8 +1584,11 @@ void btrfs_reclaim_bgs_work(struct work_
 		}
 
 next:
-		if (ret)
-			btrfs_mark_bg_to_reclaim(bg);
+		if (ret) {
+			/* Refcount held by the reclaim_bgs list after splice. */
+			btrfs_get_block_group(bg);
+			list_add_tail(&bg->bg_list, &retry_list);
+		}
 		btrfs_put_block_group(bg);
 
 		mutex_unlock(&fs_info->reclaim_bgs_lock);
@@ -1604,6 +1608,9 @@ next:
 	spin_unlock(&fs_info->unused_bgs_lock);
 	mutex_unlock(&fs_info->reclaim_bgs_lock);
 end:
+	spin_lock(&fs_info->unused_bgs_lock);
+	list_splice_tail(&retry_list, &fs_info->reclaim_bgs);
+	spin_unlock(&fs_info->unused_bgs_lock);
 	btrfs_exclop_finish(fs_info);
 	sb_end_write(fs_info->sb);
 }






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