[PATCH v3] btrfs: block-group: Fix free-space bitmap threshould

[Marcos Paulo de Souza]

From: Marcos Paulo de Souza <mpdesouza@xxxxxxxx>

[BUG]
After commit 9afc66498a0b ("btrfs: block-group: refactor how we read one
block group item"), cache->length is being assigned after calling
btrfs_create_block_group_cache. This causes a problem since
set_free_space_tree_thresholds is calculate the free-space threshould to
decide is the free-space tree should convert from extents to bitmaps.

The current code calls set_free_space_tree_thresholds with cache->length
being 0, which then makes cache->bitmap_high_thresh being zero. This
implies the system will always use bitmap instead of extents, which is
not desired if the block group is not fragmented.

This behavior can be seen by a test that expects to repair systems
with FREE_SPACE_EXTENT and FREE_SPACE_BITMAP, but the current code only
created FREE_SPACE_BITMAP.

[FIX]
Call set_free_space_tree_thresholds after setting cache->length. There
is now a WARN_ON in set_free_space_tree_thresholds to help preventing
the same mistake to happen again in the future.

Link: https://github.com/kdave/btrfs-progs/issues/251
Fixes: 9afc66498a0b ("btrfs: block-group: refactor how we read one block group item")
CC: stable@xxxxxxxxxxxxxxx # 5.8+
Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@xxxxxxxx>
---
 Changes from v2:
 * Add a WARN_ON and changed the warn message (Filipe)
 * Add a Reviewed-by tag from Filipe
 Changes from v1:
 * Add warn message (Qu)
 * Add a Reviewed-by tag from Qu

 fs/btrfs/block-group.c     | 4 +++-
 fs/btrfs/free-space-tree.c | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 44fdfa2eeb2e..01e8ba1da1d3 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1798,7 +1798,6 @@ static struct btrfs_block_group *btrfs_create_block_group_cache(
 
 	cache->fs_info = fs_info;
 	cache->full_stripe_len = btrfs_full_stripe_len(fs_info, start);
-	set_free_space_tree_thresholds(cache);
 
 	cache->discard_index = BTRFS_DISCARD_INDEX_UNUSED;
 
@@ -1908,6 +1907,8 @@ static int read_one_block_group(struct btrfs_fs_info *info,
 
 	read_block_group_item(cache, path, key);
 
+	set_free_space_tree_thresholds(cache);
+
 	if (need_clear) {
 		/*
 		 * When we mount with old space cache, we need to
@@ -2128,6 +2129,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 		return -ENOMEM;
 
 	cache->length = size;
+	set_free_space_tree_thresholds(cache);
 	cache->used = bytes_used;
 	cache->flags = type;
 	cache->last_byte_to_unpin = (u64)-1;
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c
index 8b1f5c8897b7..f072c106b82b 100644
--- a/fs/btrfs/free-space-tree.c
+++ b/fs/btrfs/free-space-tree.c
@@ -22,6 +22,10 @@ void set_free_space_tree_thresholds(struct btrfs_block_group *cache)
 	size_t bitmap_size;
 	u64 num_bitmaps, total_bitmap_size;
 
+	if (WARN_ON(cache->length == 0))
+		btrfs_warn(cache->fs_info, "block group %llu length is zero",
+						cache->start);
+
 	/*
 	 * We convert to bitmaps when the disk space required for using extents
 	 * exceeds that required for using bitmaps.
-- 
2.28.0