Re: any idea about auto export multiple btrfs snapshots?

Wang Yugui
2021-06-18T07:26:35Z

Hi,

> On Thu, 17 Jun 2021, Wang Yugui wrote:
> > > Can we go back to the beginning. What, exactly, is the problem you are
> > > trying to solve? How can you demonstrate the problem?
> > >
> > > NeilBrown
> >
> > I nfs/exported a btrfs with 2 subvols and 2 snapshot(subvol).
> >
> > # btrfs subvolume list /mnt/test
> > ID 256 gen 53 top level 5 path sub1
> > ID 260 gen 56 top level 5 path sub2
> > ID 261 gen 57 top level 5 path .snapshot/sub1-s1
> > ID 262 gen 57 top level 5 path .snapshot/sub2-s1
> >
> > and then mount.nfs4 it to /nfs/test.
> >
> > # /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'. so I though there is
> > something wrong.
>
> Certainly something is wrong. The error message implies that some
> directory is reporting the same dev an ino as an ancestor directory.
> Presumably /nfs/test and /nfs/test/sub1.
> Can you confirm that please. e.g. run the command
>
> stat /nfs/test /nfs/test/sub1
> and examine the output.

# stat /nfs/test /nfs/test/sub1
File: /nfs/test
Size: 42 Blocks: 32 IO Block: 32768 directory
Device: 36h/54d Inode: 256 Links: 1
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2021-06-18 13:50:55.409457648 +0800
Modify: 2021-06-13 10:05:10.830825901 +0800
Change: 2021-06-13 10:05:10.830825901 +0800
Birth: -
File: /nfs/test/sub1
Size: 8 Blocks: 0 IO Block: 32768 directory
Device: 36h/54d Inode: 256 Links: 1
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2021-06-18 13:51:14.463621411 +0800
Modify: 2021-06-12 21:59:10.598089917 +0800
Change: 2021-06-12 21:59:10.598089917 +0800
Birth: -

same 'Device/Inode' are reported.

but the local btrfs mount,
# stat /mnt/test/ /mnt/test/sub1
File: /mnt/test/
Size: 42 Blocks: 32 IO Block: 4096 directory
Device: 33h/51d Inode: 256 Links: 1
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2021-06-18 13:50:55.409457648 +0800
Modify: 2021-06-13 10:05:10.830825901 +0800
Change: 2021-06-13 10:05:10.830825901 +0800
Birth: -
File: /mnt/test/sub1
Size: 8 Blocks: 0 IO Block: 4096 directory
Device: 34h/52d Inode: 256 Links: 1
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2021-06-18 13:51:14.463621411 +0800
Modify: 2021-06-12 21:59:10.598089917 +0800
Change: 2021-06-12 21:59:10.598089917 +0800
Birth: -

'stat' command should cause nfs/crossmnt to happen auto, and then return
the 'stat' result?

> As sub1 is considered a different file system, it should have a
> different dev number. NFS will assign a different device number only
> when the server reports a different fsid. The Linux NFS server will
> report a different fsid if d_mountpoint() is 'true' for the dentry, and
> follow_down() results in no change the the vfsmnt,dentry in a 'struct
> path'.
>
> You have already said that d_mountpoint doesn't work for btrfs, so that
> is part of the problem. NFSD doesn't trust d_mountpoint completely as
> it only reports that the dentry is a mountpoint in some namespace, not
> necessarily in this namespace. So you really need to fix
> nfsd_mountpoint.
>
> I suggest you try adding your "dirty fix" to nfsd_mountpoint() so that
> it reports the root of a btrfs subvol as a mountpoint, and see if that
> fixes the problem. It should change the problem at least. You would
> need to get nfsd_mountpoint() to return '1' in this case, not '2'.
>
> NeilBrown

I changed the return value from 2 to 1.
if (nfsd4_is_junction(dentry))
return 1;
+ if (is_btrfs_subvol_d(dentry))
+ return 1;
if (d_mountpoint(dentry))

but the crossmnt still does not happen auto.

I tried to mount the subvol manual,
# mount.nfs4 T7610:/mnt/test/sub1 /nfs/test/sub1
mount.nfs4: Stale file handle

we add trace to is_btrfs_subvol_d(), it works as expected.
+static inline bool is_btrfs_subvol_d(const struct dentry *dentry)
+{
+ bool ret= dentry->d_inode && dentry->d_inode->i_ino == 256ULL &&
+ dentry->d_sb && dentry->d_sb->s_magic == 0x9123683E;
+ printk(KERN_INFO "is_btrfs_subvol_d(%s)=%d\n", dentry->d_name.name, ret);
+ return ret;
+}

It seems more fixes are needed.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2021/06/18