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From:   Olga Kornievskaia <aglo@umich.edu>
Date:   Wed, 19 May 2021 15:15:12 -0400
Message-ID: <CAN-5tyGgWx6F2s=t+0UAJJZEAEfNnv+Sq8eeBbnQYocKOOK8Jg@mail.gmail.com>
Subject: Re: NFSv4: Mounting NFS server which is down, blocks all other NFS
 mounts on same machine
To:     Michael Wakabayashi <mwakabayashi@vmware.com>
Cc:     "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
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On Sun, May 16, 2021 at 11:18 PM Michael Wakabayashi
<mwakabayashi@vmware.com> wrote:
>
> Hi,
>
> We're seeing what looks like an NFSv4 issue.
>
> Mounting an NFS server that is down (ping to this NFS server's IP address=
 does not respond) will block _all_ other NFS mount attempts even if the NF=
S servers are available and working properly (these subsequent mounts hang)=
.
>
> If I kill the NFS mount process that's trying to mount the dead NFS serve=
r, the NFS mounts that were blocked will immediately unblock and mount succ=
essfully, which suggests the first mount command is blocking the other moun=
t commands.
>
>
> I verified this behavior using a newly built mount.nfs command from the r=
ecent nfs-utils 2.5.3 package installed on a recent version of Ubuntu Cloud=
 Image 21.04:
> * https://sourceforge.net/projects/nfs/files/nfs-utils/2.5.3/
> * https://cloud-images.ubuntu.com/releases/hirsute/release-20210513/ubunt=
u-21.04-server-cloudimg-amd64.ova
>
>
> The reason this looks like it is specific to NFSv4 is from the following =
output showing "vers=3D4.2":
> > $ strace /sbin/mount.nfs <unreachable-IP-address>:/path /tmp/mnt
> > [ ... cut ... ]
> > mount("<unreadhable-IP-address>:/path", "/tmp/mnt", "nfs", 0, "vers=3D4=
.2,addr=3D<unreachable-IP-address>,clien"...^C^Z
>
> Also, if I try the same mount.nfs commands but specifying NFSv3, the moun=
t to the dead NFS server hangs, but the mounts to the operational NFS serve=
rs do not block and mount successfully; this bug doesn't happen when using =
NFSv3.
>
>
> We reported this issue under util-linux here:
> https://github.com/karelzak/util-linux/issues/1309
> [mounting nfs server which is down blocks all other nfs mounts on same ma=
chine #1309]
>
> I also found an older bug on this mailing list that had similar symptoms =
(but could not tell if it was the same problem or not):
> https://patchwork.kernel.org/project/linux-nfs/patch/87vaori26c.fsf@notab=
ene.neil.brown.name/
> [[PATCH/RFC] NFSv4: don't let hanging mounts block other mounts]
>
> Thanks, Mike

Hi Mike,

This is not a helpful reply but I was curious if I could reproduce
your issue but was not successful. I'm able to initiate a mount to an
unreachable-IP-address which hangs and then do another mount to an
existing server without issues. Ubuntu 21.04 seems to be 5.11 based so
I tried upstream 5.11 and I tried the latest upstream nfs-utils
(instead of what my distro has which was an older version).

To debug, perhaps get an output of the nfs4 and sunrpc tracepoints.
Or also get output from dmesg after doing =E2=80=9Cecho t >
/proc/sysrq-trigger=E2=80=9D to see where the mounts are hanging.