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

[Long Li]

On Wed, Jun 12, 2024 at 08:43:42AM -0700, Darrick J. Wong wrote:
> On Wed, Jun 12, 2024 at 07:55:31AM +0100, John Garry wrote:
> > On 12/06/2024 03:10, Long Li wrote:
> > > 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.
> > 
> > I really don't know. I'm looking through the history and it has been like
> > that this the start of my source control records.
> > 
> > Maybe it was a copy-and-paste error from XFS_FEAT_FORCEALIGN, whose value
> > has changed since.
> > 
> > Anyway, I'll ask a bit more internally, and I'll look to change to (1 << 4)
> > if ok.
> 
> I tend to use upper bits for ondisk features that are still under
> development so that (a) there won't be collisions with other features
> getting merged and (b) after the feature I'm working on gets merged, any
> old fs images in my zoo will no longer mount.
> 

I get it, thank you very much for your response.