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Date:   Wed, 23 Jun 2021 06:48:41 +0800
From:   Wang Yugui <wangyugui@e16-tech.com>
To:     Frank Filz <ffilz@redhat.com>
Subject: Re: any idea about auto export multiple btrfs snapshots?
Cc:     Frank Filz <ffilzlnx@mindspring.com>, 'NeilBrown' <neilb@suse.de>,
        linux-nfs@vger.kernel.org
In-Reply-To: <f2581d9f-dfe4-3280-80de-31683c9e647d@redhat.com>
References: <20210622064158.98CA.409509F4@e16-tech.com> <f2581d9f-dfe4-3280-80de-31683c9e647d@redhat.com>
Message-Id: <20210623064840.ABD8.409509F4@e16-tech.com>
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Hi,

> On 6/21/21 3:41 PM, Wang Yugui wrote:
> > Hi,
> >
> >> OK thanks for the information. I think they will just work in nfs-ganesha as
> >> long as the snapshots or subvols are mounted within an nfs-ganesha export or
> >> are exported explicitly. nfs-ganesha has the equivalent of knfsd's
> >> nohide/crossmnt options and when nfs-ganesha detects crossing a filesystem
> >> boundary will lookup the filesystem via getmntend and listing btrfs subvols
> >> and then expose that filesystem (via the fsid attribute) to the clients
> >> where at least the Linux nfs client will detect a filesystem boundary and
> >> create a new mount entry for it.
> >
> > Not only exported explicitly, but also kept in the same hierarchy.
> >
> > If we export
> > /mnt/test		#the btrfs
> > /mnt/test/sub1	# the btrfs subvol 1
> > /mnt/test/sub2	# the btrfs subvol 2
> >
> > we need to make sure we will not access '/mnt/test/sub1' through '/mnt/test'
> > from nfs client.
> >
> > current safe export:
> > #/mnt/test		#the btrfs, not exported
> > /mnt/test/sub1	# the btrfs subvol 1
> > /mnt/test/sub2	# the btrfs subvol 2
> >
> 
> What's the problem with exporting /mnt/test AND then exporting sub1 and sub2 as crossmnt exports? As far as I can tell, that seems to work just fine with nfs-ganesha.

I'm not sure what will happen on nfs-ganesha.

crossmnt(kernel nfsd) failed to work when exporting /mnt/test,/mnt/test/sub1,
/mnt/test/sub2.

# /bin/find /nfs/test/
/nfs/test/
find: File system loop detected; ??/nfs/test/sub1?? is part of the same file system loop as ??/nfs/test/??.
/nfs/test/.snapshot
find: File system loop detected; ??/nfs/test/.snapshot/sub1-s1?? is part of the same file system loop as ??/nfs/test/??.
find: File system loop detected; ??/nfs/test/.snapshot/sub2-s1?? is part of the same file system loop as ??/nfs/test/??.
/nfs/test/dir1
/nfs/test/dir1/a.txt
find: File system loop detected; ??/nfs/test/sub2?? is part of the same file system loop as ??/nfs/test/??

/bin/find report 'File system loop detected', it means that vfs cache(
based on st_dev + st_ino?) on nfs client side will have some problem?

In fact, I was exporting /mnt/test  for years too.
but btrfs subvols means multiple filesystems(different st_dev),  in theory, we
needs to use it based on nfs crossmnt.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2021/06/23