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Date:   Mon, 2 Dec 2019 08:05:19 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: Question about clone_range() metadata stability
Message-ID: <20191201210519.GB2418@dread.disaster.area>
References: <f063089fb62c219ea6453c7b9b0aaafd50946dae.camel@hammerspace.com>
 <20191127202136.GV6211@magnolia>
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On Wed, Nov 27, 2019 at 12:21:36PM -0800, Darrick J. Wong wrote:
> On Wed, Nov 27, 2019 at 06:38:46PM +0000, Trond Myklebust wrote:
> > Hi all
> > 
> > A quick question about clone_range() and guarantees around metadata
> > stability.
> > 
> > Are users required to call fsync/fsync_range() after calling
> > clone_range() in order to guarantee that the cloned range metadata is
> > persisted?
> 
> Yes.
> 
> > I'm assuming that it is required in order to guarantee that
> > data is persisted.
> 
> Data and metadata.  XFS and ocfs2's reflink implementations will flush
> the page cache before starting the remap, but they both require fsync to
> force the log/journal to disk.

So we need to call xfs_fs_nfs_commit_metadata() to get that done
post vfs_clone_file_range() completion on the server side, yes?

> 
> (AFAICT the same reasoning applies to btrfs, but don't trust my word for
> it.)
> 
> > I'm asking because knfsd currently just does a call to
> > vfs_clone_file_range() when parsing a NFSv4.2 CLONE operation. It does
> > not call fsync()/fsync_range() on the destination file, and since the
> > NFSv4.2 protocol does not require you to perform any other operation in
> > order to persist data/metadata, I'm worried that we may be corrupting
> > the cloned file if the NFS server crashes at the wrong moment after the
> > client has been told the clone completed.

Yup, that's exactly what server side calls to commit_metadata() are
supposed to address.

I suspect to be correct, this might require commit_metadata() to be
called on both the source and destination inodes, as both of them
may have modified metadata as a result of the clone operation. For
XFS one of them will be a no-op, but for other filesystems that
don't implement ->commit_metadata, we'll need to call
sync_inode_metadata() on both inodes...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com