linux-ext4 - Re: [PATCH 01/10] ext4: describe fast

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Date:   Tue, 3 Nov 2020 11:33:59 +0100
From:   Jan Kara <jack@...e.cz>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz
Subject: Re: [PATCH 01/10] ext4: describe fast_commit feature flags

On Sat 31-10-20 13:05:09, Harshad Shirwadkar wrote:
> Fast commit feature has flags in the file system as well in JBD2. The
> meaning of fast commit feature flags can get confusing. Update docs
> and code to add more documentation about it.
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>

Looks good to me. Thanks! You can add:

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

								Honza

> ---
>  Documentation/filesystems/ext4/journal.rst | 6 ++++++
>  Documentation/filesystems/ext4/super.rst   | 7 +++++++
>  fs/ext4/ext4.h                             | 7 +++++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/Documentation/filesystems/ext4/journal.rst b/Documentation/filesystems/ext4/journal.rst
> index 805a1e9ea3a5..849d5b119eb8 100644
> --- a/Documentation/filesystems/ext4/journal.rst
> +++ b/Documentation/filesystems/ext4/journal.rst
> @@ -256,6 +256,10 @@ which is 1024 bytes long:
>       - s\_padding2
>       -
>     * - 0x54
> +     - \_\_be32
> +     - s\_num\_fc\_blocks
> +     - Number of fast commit blocks in the journal.
> +   * - 0x58
>       - \_\_u32
>       - s\_padding[42]
>       -
> @@ -310,6 +314,8 @@ The journal incompat features are any combination of the following:
>       - This journal uses v3 of the checksum on-disk format. This is the same as
>         v2, but the journal block tag size is fixed regardless of the size of
>         block numbers. (JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3)
> +   * - 0x20
> +     - Journal has fast commit blocks. (JBD2\_FEATURE\_INCOMPAT\_FAST\_COMMIT)
>  
>  .. _jbd2_checksum_type:
>  
> diff --git a/Documentation/filesystems/ext4/super.rst b/Documentation/filesystems/ext4/super.rst
> index 93e55d7c1d40..2eb1ab20498d 100644
> --- a/Documentation/filesystems/ext4/super.rst
> +++ b/Documentation/filesystems/ext4/super.rst
> @@ -596,6 +596,13 @@ following:
>       - Sparse Super Block, v2. If this flag is set, the SB field s\_backup\_bgs
>         points to the two block groups that contain backup superblocks
>         (COMPAT\_SPARSE\_SUPER2).
> +   * - 0x400
> +     - Fast commits supported. Although fast commits blocks are
> +       backward incompatible, fast commit blocks are not always
> +       present in the journal. If fast commit blocks are present in
> +       the journal, JBD2 incompat feature
> +       (JBD2\_FEATURE\_INCOMPAT\_FAST\_COMMIT) gets
> +       set (COMPAT\_FAST\_COMMIT).
>  
>  .. _super_incompat:
>  
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 2337e443fa30..12673f9ec880 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1875,6 +1875,13 @@ static inline bool ext4_verity_in_progress(struct inode *inode)
>  #define EXT4_FEATURE_COMPAT_RESIZE_INODE	0x0010
>  #define EXT4_FEATURE_COMPAT_DIR_INDEX		0x0020
>  #define EXT4_FEATURE_COMPAT_SPARSE_SUPER2	0x0200
> +/*
> + * The reason why "FAST_COMMIT" is a compat feature is that, FS becomes
> + * incompatible only if fast commit blocks are present in the FS. Since we
> + * clear the journal (and thus the fast commit blocks), we don't mark FS as
> + * incompatible. We also have a JBD2 incompat feature, which gets set when
> + * there are fast commit blocks present in the journal.
> + */
>  #define EXT4_FEATURE_COMPAT_FAST_COMMIT		0x0400
>  #define EXT4_FEATURE_COMPAT_STABLE_INODES	0x0800
>  
> -- 
> 2.29.1.341.ge80a0c044ae-goog
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR