Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps

[Jan Kara <jack@suse.cz>]

On Mon 05-10-20 01:14:54, Josh Triplett wrote:
> Ran into an ext4 regression when testing upgrades to 5.9-rc kernels:
> 
> Commit e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in
> ext4_setup_system_zone()") breaks mounting of read-only ext4 filesystems
> with intentionally overlapping bitmap blocks.
> 
> On an always-read-only filesystem explicitly marked with
> EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS, prior to that commit, it's safe to
> point all the block and inode bitmaps to a single block of all 1s,
> because a read-only filesystem will never allocate or free any blocks or
> inodes.
> 
> However, after that commit, the block validity check rejects such
> filesystems with -EUCLEAN and "failed to initialize system zone (-117)".
> This causes systems that previously worked correctly to fail when
> upgrading to v5.9-rc2 or later.
> 
> This was obviously a bugfix, and I'm not suggesting that it should be
> reverted; it looks like this effectively worked by accident before,
> because the block_validity check wasn't fully functional. However, this
> does break real systems, and I'd like to get some kind of regression fix
> in before 5.9 final if possible. I think it would suffice to make
> block_validity default to false if and only if
> EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is set.
> 
> Does that seem like a reasonable fix?
> 
> Here's a quick sketch of a patch, which I've tested and confirmed to
> work:
> 
> ----- 8< -----
> Subject: [PATCH] Fix ext4 regression in v5.9-rc2 on ro fs with overlapped bitmaps
> 
> Commit e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in
> ext4_setup_system_zone()") breaks mounting of read-only ext4 filesystems
> with intentionally overlapping bitmap blocks.
> 
> On an always-read-only filesystem explicitly marked with
> EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS, prior to that commit, it's safe to
> point all the block and inode bitmaps to a single block of all 1s,
> because a read-only filesystem will never allocate or free any blocks or
> inodes.
> 
> However, after that commit, the block validity check rejects such
> filesystems with -EUCLEAN and "failed to initialize system zone (-117)".
> This causes systems that previously worked correctly to fail when
> upgrading to v5.9-rc2 or later.
> 
> Fix this by defaulting block_validity to off when
> EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is set.
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> Fixes: e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in ext4_setup_system_zone()")

The patch looks fine to me. Thanks for fixing this and for educating me
about the feature :) You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza


> ---
>  fs/ext4/ext4.h  | 2 ++
>  fs/ext4/super.c | 3 ++-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 523e00d7b392..7874028fa864 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1834,6 +1834,7 @@ static inline bool ext4_verity_in_progress(struct inode *inode)
>  #define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM	0x0400
>  #define EXT4_FEATURE_RO_COMPAT_READONLY		0x1000
>  #define EXT4_FEATURE_RO_COMPAT_PROJECT		0x2000
> +#define EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS	0x4000
>  #define EXT4_FEATURE_RO_COMPAT_VERITY		0x8000
>  
>  #define EXT4_FEATURE_INCOMPAT_COMPRESSION	0x0001
> @@ -1930,6 +1931,7 @@ EXT4_FEATURE_RO_COMPAT_FUNCS(bigalloc,		BIGALLOC)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(metadata_csum,	METADATA_CSUM)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(readonly,		READONLY)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(project,		PROJECT)
> +EXT4_FEATURE_RO_COMPAT_FUNCS(shared_blocks,	SHARED_BLOCKS)
>  EXT4_FEATURE_RO_COMPAT_FUNCS(verity,		VERITY)
>  
>  EXT4_FEATURE_INCOMPAT_FUNCS(compression,	COMPRESSION)
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index ea425b49b345..f57a7e966e44 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3954,7 +3954,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  	else
>  		set_opt(sb, ERRORS_RO);
>  	/* block_validity enabled by default; disable with noblock_validity */
> -	set_opt(sb, BLOCK_VALIDITY);
> +	if (!ext4_has_feature_shared_blocks(sb))
> +		set_opt(sb, BLOCK_VALIDITY);
>  	if (def_mount_opts & EXT4_DEFM_DISCARD)
>  		set_opt(sb, DISCARD);
>  
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR