Re: [PATCH AUTOSEL 6.0 41/46] btrfs: relax block-group-tree feature dependency checks

[David Sterba]

On Tue, Oct 11, 2022 at 10:50:09AM -0400, Sasha Levin wrote:
> From: Qu Wenruo <wqu@xxxxxxxx>
> 
> [ Upstream commit d7f67ac9a928fa158a95573406eac0a887bbc28c ]
> 
> [BUG]
> When one user did a wrong attempt to clear block group tree, which can
> not be done through mount option, by using "-o clear_cache,space_cache=v2",
> it will cause the following error on a fs with block-group-tree feature:
> 
>   BTRFS info (device dm-1): force clearing of disk cache
>   BTRFS info (device dm-1): using free space tree
>   BTRFS info (device dm-1): clearing free space tree
>   BTRFS info (device dm-1): clearing compat-ro feature flag for FREE_SPACE_TREE (0x1)
>   BTRFS info (device dm-1): clearing compat-ro feature flag for FREE_SPACE_TREE_VALID (0x2)
>   BTRFS error (device dm-1): block-group-tree feature requires fres-space-tree and no-holes
>   BTRFS error (device dm-1): super block corruption detected before writing it to disk
>   BTRFS: error (device dm-1) in write_all_supers:4318: errno=-117 Filesystem corrupted (unexpected superblock corruption detected)
>   BTRFS warning (device dm-1: state E): Skipping commit of aborted transaction.
> 
> [CAUSE]
> Although the dependency for block-group-tree feature is just an
> artificial one (to reduce test matrix), we put the dependency check into
> btrfs_validate_super().
> 
> This is too strict, and during space cache clearing, we will have a
> window where free space tree is cleared, and we need to commit the super
> block.
> 
> In that window, we had block group tree without v2 cache, and triggered
> the artificial dependency check.
> 
> This is not necessary at all, especially for such a soft dependency.
> 
> [FIX]
> Introduce a new helper, btrfs_check_features(), to do all the runtime
> limitation checks, including:
> 
> - Unsupported incompat flags check
> 
> - Unsupported compat RO flags check
> 
> - Setting missing incompat flags
> 
> - Artificial feature dependency checks
>   Currently only block group tree will rely on this.
> 
> - Subpage runtime check for v1 cache
> 
> With this helper, we can move quite some checks from
> open_ctree()/btrfs_remount() into it, and just call it after
> btrfs_parse_options().
> 
> Now "-o clear_cache,space_cache=v2" will not trigger the above error
> anymore.
> 
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> Reviewed-by: David Sterba <dsterba@xxxxxxxx>
> [ edit messages ]
> Signed-off-by: David Sterba <dsterba@xxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

Please drop this from the stable queue, it's for an unreleased feature
(ETA 6.1).