"> It's limited to xfs, no failure on ext4 to date", this is incorrect. I
have been able to reproduce this issue with ext4. In order to do that, I need to run the
full test (on both pmems in the system) and not the half test (only 1 pmem) that I use for
inducing the hang under XFS. The test also runs considerably longer before failing with
ext4 than XFS.
From: Dave Chinner [mailto:firstname.lastname@example.org]
Sent: Monday, March 11, 2019 9:38 PM
To: Williams, Dan J <dan.j.williams(a)intel.com>
Cc: Matthew Wilcox <willy(a)infradead.org>; Linux MM <linux-mm(a)kvack.org>;
linux-nvdimm <linux-nvdimm(a)lists.01.org>; linux-fsdevel
<linux-fsdevel(a)vger.kernel.org>; Barror, Robert <robert.barror(a)intel.com>
Subject: Re: Hang / zombie process from Xarray page-fault conversion (bisected)
On Mon, Mar 11, 2019 at 08:35:05PM -0700, Dan Williams wrote:
On Mon, Mar 11, 2019 at 8:10 AM Matthew Wilcox
> 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/$pid/stack for a hung task?
It's fairly easy to reproduce, I'll see if I can package up all the
dependencies into something that fails in a VM.
It's limited to xfs, no failure on ext4 to date.
The hung process appears to be:
That's completely internal to XFS. Every 30s the work is triggered and it either does
a log flush (if the fs is active) or it syncs the superblock to clean the log and idle the
filesystem. It has nothing to do with user processes, and I don't see why killing a
process has any effect on what it does...
...and then the rest of the database processes grind to a halt from
Robert was kind enough to capture /proc/$pid/stack, but nothing interesting:
Much more useful would be:
# echo w > /proc/sysrq-trigger
And post the entire output of dmesg.