Re: what on earth is going on here? paths above mountpoints turn into "(unreachable)"

[bfields@fieldses.org (J. Bruce Fields)]

On Tue, Feb 03, 2015 at 12:25:18AM +0000, Nix wrote:
> This is with client and server both running NFSv3 on 3.18.4 (an upgrade
> to .5 is on the cards soon). (This is *not* the same client I've been
> reporting the panic on, though the server is the same, and this client
> too has been seeing the panic. Not that that's relevant, since that's a
> bug on shutdown, and this isn't. Indeed this may not be a bug at all,
> but it's new behaviour with 3.18.x, and it breaks programs, and it's
> weird.)
> 
> The server says (for this client):
> 
> /usr/archive -fsid=25,root_squash,async,subtree_check,crossmnt mutilate(rw,root_squash,insecure)
> /usr/archive/series -fsid=29,root_squash,async,subtree_check mutilate(rw,root_squash,insecure)
> 
> The client says:
> 
> package.srvr.nix:/usr/archive /usr/archive nfs defaults,rw
> 
> (i.e. relying on the crossmnt, though I have just changed this to
> explicitly mount both mount points, while preserving the crossmnt on the
> parent for the sake of other clients that can't mount both because
> they're using libnfs and need to follow symlinks from other places under
> /usr/archive/ into /usr/archive/series, and the software is too stupid
> to understand that there might be more than one mount point involved.)
> 
> I'm seeing this bizarre output after a long delay (memory pressure not
> required: vapoursynth, which was running here, is using a couple of gig
> out of 16GiB, and the machine has 12GiB in buffers/cache):
> 
> FileNotFoundError: [Errno 2] No such file or directory: '(unreachable)/Orphan-Black/1'

Haven't really read this carefully, just noticed the ENOENT.  There was
49a068f82a "rpc: fix xdr_truncate_encode to handle buffer ending on page
boundary" recently, fixing a problem introduced in 3.16 that could I
think cause an enoent if Orphan-Black/ was a large-ish directory.

Actually, that fix is in 3.18.3, never mind....

--b.

> Failed to retrieve output node. Invalid index specified?
> pipe:: Invalid data found when processing input
> nix@mutilate 45 .../Orphan-Black/1% pwd -P
> /usr/archive/series/Orphan-Black/1
> # Well, doesn't this cwd look weird.
> nix@mutilate 46 ...//1% ls -l /proc/self/cwd
> lrwxrwxrwx 1 nix users 0 Feb  2 23:28 /proc/self/cwd -> /Orphan-Black/1
> nix@mutilate 49 .../Orphan-Black/1% ls -id .
> 624194 .
> # Try going out of the directory and back into it again.
> nix@mutilate 50 .../Orphan-Black/1% cd -
> /tmp
> nix@mutilate 51 /tmp% cd -
> /usr/archive/series/Orphan-Black/1
> # Same inode, but now the cwd is valid!
> nix@mutilate 52 .../Orphan-Black/1% ls -id .
> 624194 .
> nix@mutilate 54 .../Orphan-Black/1% ls -l /proc/self/cwd
> lrwxrwxrwx 1 nix users 0 Feb  2 23:28 /proc/self/cwd -> /usr/archive/series/Orphan-Black/1
> 
> So something about the mountpoint is expiring away, as if it had been
> umount -l'ed. No automounter of any kind is running, and (obviously) it
> *wasn't* umount -l'ed.
> 
> I guess by tomorrow I'll know if this is crossmnt-related, at least... I
> know crossmnt is considered bad and evil: is this sort of thing why?
> 
> The filesystem being NFS-exported is on a USB-attached disk that spins
> down when not in use, but I'd not expect that to do anything other than
> cause delays on first access (and indeed even if I turn off spindown
> this still happens, even though the filesystem is perfectly accessible,
> from the server and indeed from the client: it's just... disconnected,
> which seems to greatly annoy a lot of programs.)
> 
> If it happens again I'll see if I can arrange to have two processes, one
> with a cwd in the full path, one with a cwd in the truncated one. That
> at least will tell us whether this is some sort of expiry thing attached
> to one mountpoint, and going back into it is de-expiring it for all
> users under that mountpoint, or whether we really are seeing a new mount
> here, somehow (how, without mount(2) ever being called?!).
> 
> -- 
> NULL && (void)
> --
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