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Date:   Mon, 2 Aug 2021 20:15:46 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        Linux NFS list <linux-nfs@vger.kernel.org>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: A Third perspective on BTRFS nfsd subvol dev/inode number issues.
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On Tue, Aug 03, 2021 at 08:36:44AM +1000, NeilBrown wrote:
> On Tue, 03 Aug 2021, J. Bruce Fields wrote:
> > On Tue, Aug 03, 2021 at 07:59:30AM +1000, NeilBrown wrote:
> > > On Tue, 03 Aug 2021, J. Bruce Fields wrote:
> > > > On Tue, Aug 03, 2021 at 07:10:44AM +1000, NeilBrown wrote:
> > > > > On Mon, 02 Aug 2021, J. Bruce Fields wrote:
> > > > > > On Mon, Aug 02, 2021 at 02:18:29PM +1000, NeilBrown wrote:
> > > > > > > For btrfs, the "location" is root.objectid ++ file.objectid.  I think
> > > > > > > the inode should become (file.objectid ^ swab64(root.objectid)).  This
> > > > > > > will provide numbers that are unique until you get very large subvols,
> > > > > > > and very many subvols.
> > > > > > 
> > > > > > If you snapshot a filesystem, I'd expect, at least by default, that
> > > > > > inodes in the snapshot to stay the same as in the snapshotted
> > > > > > filesystem.
> > > > > 
> > > > > As I said: we need to challenge and revise user-space (and meat-space)
> > > > > expectations. 
> > > > 
> > > > The example that came to mind is people that export a snapshot, then
> > > > replace it with an updated snapshot, and expect that to be transparent
> > > > to clients.
> > > > 
> > > > Our client will error out with ESTALE if it notices an inode number
> > > > changed out from under it.
> > > 
> > > Will it?
> > 
> > See fs/nfs/inode.c:nfs_check_inode_attributes():
> > 
> > 	if (nfsi->fileid != fattr->fileid) {
> >                 /* Is this perhaps the mounted-on fileid? */
> >                 if ((fattr->valid & NFS_ATTR_FATTR_MOUNTED_ON_FILEID) &&
> >                     nfsi->fileid == fattr->mounted_on_fileid)
> >                         return 0;
> >                 return -ESTALE;
> >         }
> 
> That code fires if the fileid (inode number) reported for a particular
> filehandle changes.  I'm saying that won't happen.
> 
> If you reflink (aka snaphot) a btrfs subtree (aka "subvol"), then the
> new sub-tree will ALREADY have different filehandles than the original
> subvol.

Whoops, you're right, sorry for the noise....

--b.

> Whether it has the same inode numbers or different ones is
> irrelevant to NFS.