Re: [PATCH 1/1] xfs: introduce new file range commit ioctls

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-08-23 at 10:41 -0700, Darrick J. Wong wrote:
> On Fri, Aug 23, 2024 at 09:20:15AM -0400, Jeff Layton wrote:
> > On Thu, 2024-08-22 at 21:12 -0700, Christoph Hellwig wrote:
> > > On Thu, Aug 22, 2024 at 05:01:22PM -0700, Darrick J. Wong wrote:
> > > > From: Darrick J. Wong <djwong@kernel.org>
> > > > 
> > > > This patch introduces two more new ioctls to manage atomic
> > > > updates to
> > > > file contents -- XFS_IOC_START_COMMIT and
> > > > XFS_IOC_COMMIT_RANGE.  The
> > > > commit mechanism here is exactly the same as what
> > > > XFS_IOC_EXCHANGE_RANGE
> > > > does, but with the additional requirement that file2 cannot
> > > > have changed
> > > > since some sampling point.  The start-commit ioctl performs the
> > > > sampling
> > > > of file attributes.
> > > 
> > > The code itself looks simply enough now, but how do we guarantee
> > > that ctime actually works as a full change count and not just by
> > > chance here?
> > > 
> > 
> > With current mainline kernels it won't, but the updated multigrain
> > timestamp series is in linux-next and is slated to go into v6.12.
> > At
> > that point it should be fine for this purpose.
> 
> <nod> If these both get merged for 6.12, I think the appropriate port
> for this patch is to change xfs_ioc_start_commit to do:
> 
> 	struct kstat	kstat;
> 
> 	fill_mg_cmtime(&kstat, STATX_CTIME | STATX_MTIME,
> XFS_I(ip2));
> 	kern_f->file2_ctime		= kstat.ctime.tv_sec;
> 	kern_f->file2_ctime_nsec	= kstat.ctime.tv_nsec;
> 	kern_f->file2_mtime		= kstat.mtime.tv_sec;
> 	kern_f->file2_mtime_nsec	= kstat.mtime.tv_nsec;
> 

Yep, that's exactly what you'd want to do.

> instead of open-coding the inode_get_[cm]time calls.  The entire
> exchangerange feature is still marked experimental, so I didn't think
> it
> was worth rebasing my entire dev branch on the multigrain timestamp
> redux series; we can just fix it later.
> 

Sounds good.
-- 
Jeff Layton <jlayton@kernel.org>