Re: [PATCH v4 6/9] xfs: switch to multigrain timestamps

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-07-08 at 14:51 -0400, Jeff Layton wrote:
> On Mon, 2024-07-08 at 11:47 -0700, Darrick J. Wong wrote:
> > On Mon, Jul 08, 2024 at 11:53:39AM -0400, Jeff Layton wrote:
> > > Enable multigrain timestamps, which should ensure that there is an
> > > apparent change to the timestamp whenever it has been written after
> > > being actively observed via getattr.
> > > 
> > > Also, anytime the mtime changes, the ctime must also change, and those
> > > are now the only two options for xfs_trans_ichgtime. Have that function
> > > unconditionally bump the ctime, and ASSERT that XFS_ICHGTIME_CHG is
> > > always set.
> > > 
> > > Finally, stop setting STATX_CHANGE_COOKIE in getattr, since the ctime
> > > should give us better semantics now.
> > > 
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > >  fs/xfs/libxfs/xfs_trans_inode.c |  6 +++---
> > >  fs/xfs/xfs_iops.c               | 10 +++-------
> > >  fs/xfs/xfs_super.c              |  2 +-
> > >  3 files changed, 7 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
> > > index 69fc5b981352..1f3639bbf5f0 100644
> > > --- a/fs/xfs/libxfs/xfs_trans_inode.c
> > > +++ b/fs/xfs/libxfs/xfs_trans_inode.c
> > > @@ -62,12 +62,12 @@ xfs_trans_ichgtime(
> > >  	ASSERT(tp);
> > >  	xfs_assert_ilocked(ip, XFS_ILOCK_EXCL);
> > >  
> > > -	tv = current_time(inode);
> > > +	/* If the mtime changes, then ctime must also change */
> > > +	ASSERT(flags & XFS_ICHGTIME_CHG);
> > >  
> > > +	tv = inode_set_ctime_current(inode);
> > >  	if (flags & XFS_ICHGTIME_MOD)
> > >  		inode_set_mtime_to_ts(inode, tv);
> > > -	if (flags & XFS_ICHGTIME_CHG)
> > > -		inode_set_ctime_to_ts(inode, tv);
> > >  	if (flags & XFS_ICHGTIME_CREATE)
> > >  		ip->i_crtime = tv;
> > >  }
> > > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> > > index a00dcbc77e12..d25872f818fa 100644
> > > --- a/fs/xfs/xfs_iops.c
> > > +++ b/fs/xfs/xfs_iops.c
> > > @@ -592,8 +592,9 @@ xfs_vn_getattr(
> > >  	stat->gid = vfsgid_into_kgid(vfsgid);
> > >  	stat->ino = ip->i_ino;
> > >  	stat->atime = inode_get_atime(inode);
> > > -	stat->mtime = inode_get_mtime(inode);
> > > -	stat->ctime = inode_get_ctime(inode);
> > > +
> > > +	fill_mg_cmtime(stat, request_mask, inode);
> > 
> > Sooo... for setting up a commit-range operation[1], XFS_IOC_START_COMMIT
> > could populate its freshness data by calling:
> > 
> > 	struct kstat dummy;
> > 
> > 	fill_mg_ctime(&dummy, STATX_CTIME | STATX_MTIME, inode);
> > 
> > and then using dummy.[cm]time to populate the freshness data that it
> > gives to userspace, right?  Having set QUERIED, a write to the file
> > immediately afterwards will cause a (tiny) increase in ctime_nsec which
> > will cause the XFS_IOC_COMMIT_RANGE to reject the commit[2].  Right?
> > 
> 
> Yes. Once you call fill_mg_ctime, the first write after that point
> should cause the kernel to ensure that there is a distinct change in
> the ctime.
> 
> IOW, I think this should alleviate the concerns I had before with using
> timestamps with the XFS_IOC_COMMIT_RANGE interface.
> 
> 

Oh, and to be clear, if you're _only_ worried about changes to the
contents of the file (and not the metadata), you should be able to do
this instead:

    fill_mg_ctime(&dummy, STATX_MTIME, inode);

...and that should avoid false positives from metadata-only changes.

Querying only the mtime still causes the QUERIED flag to be set, and
the kernel to give you distinct timestamps.

> > --D
> > 
> > [1] https://lore.kernel.org/linux-xfs/20240227174649.GL6184@frogsfrogsfrogs/
> > [2] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=atomic-file-commits&id=0520d89c2698874c1f56ddf52ec4b8a3595baa14
> > 
> > > +
> > >  	stat->blocks = XFS_FSB_TO_BB(mp, ip->i_nblocks + ip->i_delayed_blks);
> > >  
> > >  	if (xfs_has_v3inodes(mp)) {
> > > @@ -603,11 +604,6 @@ xfs_vn_getattr(
> > >  		}
> > >  	}
> > >  
> > > -	if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
> > > -		stat->change_cookie = inode_query_iversion(inode);
> > > -		stat->result_mask |= STATX_CHANGE_COOKIE;
> > > -	}
> > > -
> > >  	/*
> > >  	 * Note: If you add another clause to set an attribute flag, please
> > >  	 * update attributes_mask below.
> > > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > > index 27e9f749c4c7..210481b03fdb 100644
> > > --- a/fs/xfs/xfs_super.c
> > > +++ b/fs/xfs/xfs_super.c
> > > @@ -2052,7 +2052,7 @@ static struct file_system_type xfs_fs_type = {
> > >  	.init_fs_context	= xfs_init_fs_context,
> > >  	.parameters		= xfs_fs_parameters,
> > >  	.kill_sb		= xfs_kill_sb,
> > > -	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
> > > +	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME,
> > >  };
> > >  MODULE_ALIAS_FS("xfs");
> > >  
> > > 
> > > -- 
> > > 2.45.2
> > > 
> > > 
> 

-- 
Jeff Layton <jlayton@kernel.org>