Re: Hang / zombie process from Xarray page-fault conversion (bisected)

From: Dan Williams <dan.j.williams@intel.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Linux MM <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"Barror, Robert" <robert.barror@intel.com>
Subject: Re: Hang / zombie process from Xarray page-fault conversion (bisected)
Date: Thu, 14 Mar 2019 00:34:51 -0700	[thread overview]
Message-ID: <CAPcyv4i+z0RT7rTw+4w-h8dOyscVk1g3F+cu2pKHqqJjTgU++A@mail.gmail.com> (raw)
In-Reply-To: <20190312043754.GD23020@dastard>

On Mon, Mar 11, 2019 at 9:38 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Mon, Mar 11, 2019 at 08:35:05PM -0700, Dan Williams wrote:
> > On Mon, Mar 11, 2019 at 8:10 AM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Thu, Mar 07, 2019 at 10:16:17PM -0800, Dan Williams wrote:
> > > > Hi Willy,
> > > >
> > > > We're seeing a case where RocksDB hangs and becomes defunct when
> > > > trying to kill the process. v4.19 succeeds and v4.20 fails. Robert was
> > > > able to bisect this to commit b15cd800682f "dax: Convert page fault
> > > > handlers to XArray".
> > > >
> > > > I see some direct usage of xa_index and wonder if there are some more
> > > > pmd fixups to do?
> > > >
> > > > Other thoughts?
> > >
> > > I don't see why killing a process would have much to do with PMD
> > > misalignment.  The symptoms (hanging on a signal) smell much more like
> > > leaving a locked entry in the tree.  Is this easy to reproduce?  Can you
> > > get /proc/$pid/stack for a hung task?
> >
> > It's fairly easy to reproduce, I'll see if I can package up all the
> > dependencies into something that fails in a VM.
> >
> > It's limited to xfs, no failure on ext4 to date.
> >
> > The hung process appears to be:
> >
> >      kworker/53:1-xfs-sync/pmem0
>
> That's completely internal to XFS. Every 30s the work is triggered
> and it either does a log flush (if the fs is active) or it syncs the
> superblock to clean the log and idle the filesystem. It has nothing
> to do with user processes, and I don't see why killing a process has
> any effect on what it does...
>
> > ...and then the rest of the database processes grind to a halt from there.
> >
> > Robert was kind enough to capture /proc/$pid/stack, but nothing interesting:
> >
> > [<0>] worker_thread+0xb2/0x380
> > [<0>] kthread+0x112/0x130
> > [<0>] ret_from_fork+0x1f/0x40
> > [<0>] 0xffffffffffffffff
>
> Much more useful would be:
>
> # echo w > /proc/sysrq-trigger
>
> And post the entire output of dmesg.

Here it is:

https://gist.github.com/djbw/ca7117023305f325aca6f8ef30e11556

There are some process stuck indefinitely waiting to acquire an Xarray
entry lock.