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From:   Igor Raits <igor@gooddata.com>
Date:   Tue, 23 Aug 2022 14:26:22 +0200
Message-ID: <CA+9S74je9Ack6c-H_a6raYFJSMvdSkjP76_05xzeT4wkut9DCA@mail.gmail.com>
Subject: Re: Regression: deadlock in io_schedule / nfs_writepage_locked
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "anna@kernel.org" <anna@kernel.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
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Hello Trond,

On Mon, Aug 22, 2022 at 5:01 PM Trond Myklebust <trondmy@hammerspace.com> w=
rote:
>
> On Mon, 2022-08-22 at 16:43 +0200, Igor Raits wrote:
> > [You don't often get email from igor@gooddata.com. Learn why this is
> > important at https://aka.ms/LearnAboutSenderIdentification ]
> >
> > Hello Trond,
> >
> > On Mon, Aug 22, 2022 at 4:02 PM Trond Myklebust
> > <trondmy@hammerspace.com> wrote:
> > >
> > > On Mon, 2022-08-22 at 10:16 +0200, Igor Raits wrote:
> > > > [You don't often get email from igor@gooddata.com. Learn why this
> > > > is
> > > > important at https://aka.ms/LearnAboutSenderIdentification ]
> > > >
> > > > Hello everyone,
> > > >
> > > > Hopefully I'm sending this to the right place=E2=80=A6
> > > > We recently started to see the following stacktrace quite often
> > > > on
> > > > our
> > > > VMs that are using NFS extensively (I think after upgrading to
> > > > 5.18.11+, but not sure when exactly. For sure it happens on
> > > > 5.18.15):
> > > >
> > > > INFO: task kworker/u36:10:377691 blocked for more than 122
> > > > seconds.
> > > >      Tainted: G            E     5.18.15-1.gdc.el8.x86_64 #1
> > > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > > message.
> > > > task:kworker/u36:10  state:D stack:    0 pid:377691 ppid:     2
> > > > flags:0x00004000
> > > > Workqueue: writeback wb_workfn (flush-0:308)
> > > > Call Trace:
> > > > <TASK>
> > > > __schedule+0x38c/0x7d0
> > > > schedule+0x41/0xb0
> > > > io_schedule+0x12/0x40
> > > > __folio_lock+0x110/0x260
> > > > ? filemap_alloc_folio+0x90/0x90
> > > > write_cache_pages+0x1e3/0x4d0
> > > > ? nfs_writepage_locked+0x1d0/0x1d0 [nfs]
> > > > nfs_writepages+0xe1/0x200 [nfs]
> > > > do_writepages+0xd2/0x1b0
> > > > ? check_preempt_curr+0x47/0x70
> > > > ? ttwu_do_wakeup+0x17/0x180
> > > > __writeback_single_inode+0x41/0x360
> > > > writeback_sb_inodes+0x1f0/0x460
> > > > __writeback_inodes_wb+0x5f/0xd0
> > > > wb_writeback+0x235/0x2d0
> > > > wb_workfn+0x348/0x4a0
> > > > ? put_prev_task_fair+0x1b/0x30
> > > > ? pick_next_task+0x84/0x940
> > > > ? __update_idle_core+0x1b/0xb0
> > > > process_one_work+0x1c5/0x390
> > > > worker_thread+0x30/0x360
> > > > ? process_one_work+0x390/0x390
> > > > kthread+0xd7/0x100
> > > > ? kthread_complete_and_exit+0x20/0x20
> > > > ret_from_fork+0x1f/0x30
> > > > </TASK>
> > > >
> > > > I see that something very similar was fixed in btrfs
> > > > (
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/co=
mmi
> > > > t/?h=3Dlinux-5.18.y&id=3D9535ec371d741fa037e37eddc0a5b25ba82d0027)
> > > > but I could not find anything similar for NFS.
> > > >
> > > > Do you happen to know if this is already fixed? If so, would you
> > > > mind
> > > > sharing some commits? If not, could you help getting this
> > > > addressed?
> > > >
> > >
> > > The stack trace you show above isn't particularly helpful for
> > > diagnosing what the problem is.
> > >
> > > All it is saying is that 'thread A' is waiting to take a page lock
> > > that
> > > is being held by a different 'thread B'. Without information on
> > > what
> > > 'thread B' is doing, and why it isn't releasing the lock, there is
> > > nothing we can conclude.
> >
> > Do you have some hint how to debug this issue further (when it
> > happens
> > again)? Would `virsh dump` to get a memory dump and then some kind of
> > "bt all" via crash help to get more information?
> > Or something else?
> >
> > Thanks in advance!
> > --
> > Igor Raits
>
> Please try running the following two lines of 'bash' script as root:
>
> (for tt in $(grep -l 'nfs[^d]' /proc/*/stack); do echo "${tt}:"; cat ${tt=
}; echo; done) >/tmp/nfs_threads.txt
>
> cat /sys/kernel/debug/sunrpc/rpc_clnt/*/tasks > /tmp/rpc_tasks.txt
>
> and then send us the output from the two files /tmp/nfs_threads.txt and
> /tmp/rpc_tasks.txt.
>
> The file nfs_threads.txt gives us a full set of stack traces from all
> processes that are currently in the NFS client code. So it should
> contain both the stack trace from your 'thread A' above, and the traces
> from all candidates for the 'thread B' process that is causing the
> blockage.
> The file rpc_tasks.txt gives us the status of any RPC calls that might
> be outstanding and might help diagnose any issues with the TCP
> connection.
>
> That should therefore give us a better starting point for root causing
> the problem.

We just hit the problem again and here are the files (copying in-line
as the output is not very large):

22/08/23 13:11:37 PD277054 rack-na3/k8s_worker, Cluster 53 @ rack-na3 (id 5=
3)
[root@na3-k8s-worker03:~] cat /tmp/rpc_tasks.txt

22/08/23 13:11:43 PD277054 rack-na3/k8s_worker, Cluster 53 @ rack-na3 (id 5=
3)
[root@na3-k8s-worker03:~] cat /tmp/nfs_threads.txt
/proc/3093002/stack:
[<0>] __folio_lock+0x110/0x260
[<0>] write_cache_pages+0x1e3/0x4d0
[<0>] nfs_writepages+0xe1/0x200 [nfs]
[<0>] do_writepages+0xd2/0x1b0
[<0>] __writeback_single_inode+0x41/0x360
[<0>] writeback_sb_inodes+0x1f0/0x460
[<0>] __writeback_inodes_wb+0x5f/0xd0
[<0>] wb_writeback+0x235/0x2d0
[<0>] wb_workfn+0x312/0x4a0
[<0>] process_one_work+0x1c5/0x390
[<0>] worker_thread+0x30/0x360
[<0>] kthread+0xd7/0x100
[<0>] ret_from_fork+0x1f/0x30

/proc/89011/stack:
[<0>] nfs41_callback_svc+0x1af/0x1c0 [nfsv4]
[<0>] kthread+0xd7/0x100
[<0>] ret_from_fork+0x1f/0x30

>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>


--=20
Igor Raits