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btrfs: add comment about list_is_singular() use at btrfs_delete_unused_bgs()

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At btrfs_delete_unused_bgs(), the use of the list_is_singular() check on
a block group may not be immediately obvious. It is there to prevent
losing raid profile information for a block group type (data, metadata or
system), as that information is removed from
fs_info->avail_[data|metadata|system]_alloc_bits when the last block group
of a given type is deleted. So deleting the block group would later result
in creating block groups of that type with a single profile (because
fs_info->avail_*_alloc_bits would have a value of 0).

This check was added in commit aefbe9a633 ("btrfs: Fix lost-data-profile
caused by auto removing bg").

So add a comment mentioning the need for the check.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2024-01-25 09:53:26 +00:00 committed by David Sterba
parent 4d9450113e
commit edebd19a4a
1 changed files with 7 additions and 0 deletions
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7
fs/btrfs/block-group.c
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@ -1522,6 +1522,13 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
* outstanding allocations in this block group. We do
* the ro check in case balance is currently acting on
* this block group.
*
* Also bail out if this is the only block group for its
* type, because otherwise we would lose profile
* information from fs_info->avail_*_alloc_bits and the
* next block group of this type would be created with a
* "single" profile (even if we're in a raid fs) because
* fs_info->avail_*_alloc_bits would be 0.
*/
trace_btrfs_skip_unused_block_group(block_group);
spin_unlock(&block_group->lock);