Re: [PATCH v3 08/21] xfs: Introduce FORCEALIGN inode flag

[Long Li]

On Mon, Apr 29, 2024 at 05:47:33PM +0000, John Garry wrote:
> From: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> 
> Add a new inode flag to require that all file data extent mappings must
> be aligned (both the file offset range and the allocated space itself)
> to the extent size hint.  Having a separate COW extent size hint is no
> longer allowed.
> 
> The goal here is to enable sysadmins and users to mandate that all space
> mappings in a file must have a startoff/blockcount that are aligned to
> (say) a 2MB alignment and that the startblock/blockcount will follow the
> same alignment.
> 
> jpg: Enforce extsize is a power-of-2 and aligned with afgsize + stripe
>      alignment for forcealign
> Signed-off-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Co-developed-by: John Garry <john.g.garry@xxxxxxxxxx>
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
>  fs/xfs/libxfs/xfs_format.h    |  6 ++++-
>  fs/xfs/libxfs/xfs_inode_buf.c | 50 +++++++++++++++++++++++++++++++++++
>  fs/xfs/libxfs/xfs_inode_buf.h |  3 +++
>  fs/xfs/libxfs/xfs_sb.c        |  2 ++
>  fs/xfs/xfs_inode.c            | 12 +++++++++
>  fs/xfs/xfs_inode.h            |  2 +-
>  fs/xfs/xfs_ioctl.c            | 34 +++++++++++++++++++++++-
>  fs/xfs/xfs_mount.h            |  2 ++
>  fs/xfs/xfs_super.c            |  4 +++
>  include/uapi/linux/fs.h       |  2 ++
>  10 files changed, 114 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index 2b2f9050fbfb..4dd295b047f8 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -353,6 +353,7 @@ xfs_sb_has_compat_feature(
>  #define XFS_SB_FEAT_RO_COMPAT_RMAPBT   (1 << 1)		/* reverse map btree */
>  #define XFS_SB_FEAT_RO_COMPAT_REFLINK  (1 << 2)		/* reflinked files */
>  #define XFS_SB_FEAT_RO_COMPAT_INOBTCNT (1 << 3)		/* inobt block counts */
> +#define XFS_SB_FEAT_RO_COMPAT_FORCEALIGN (1 << 30)	/* aligned file data extents */
 
Hi, John

You know I've been using and testing your atomic writes patch series recently,
and I'm particularly interested in the changes to the on-disk format. I noticed
that XFS_SB_FEAT_RO_COMPAT_FORCEALIGN uses bit 30 instead of bit 4, which would
be the next available bit in sequence.

I'm wondering if using bit 30 is just a temporary solution to avoid conflicts, 
and if the plan is to eventually use bits sequentially, for example, using bit 4?
I'm looking forward to your explanation.

Thanks,
Long Li