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

[David Sterba]

On Fri, Aug 21, 2020 at 11:54:44AM -0300, Marcos Paulo de Souza wrote:
> 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>

Added to misc-next, thanks.