linux-ext4 - Re: xfstests generic/130 hang with non-4k block size ext4 on 4.7-rc1 kernel

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Date:	Mon, 20 Jun 2016 13:39:50 +0200
From:	Jan Kara <jack@...e.cz>
To:	Nikola Pajkovsky <nikola.pajkovsky@...ddata.com>
Cc:	Jan Kara <jack@...e.cz>,
	Holger Hoffstätte 
	<holger@...lied-asynchrony.com>, linux-ext4@...r.kernel.org,
	Jan Kara <jack@...e.com>
Subject: Re: xfstests generic/130 hang with non-4k block size ext4 on 4.7-rc1
 kernel

On Thu 16-06-16 16:42:58, Nikola Pajkovsky wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> > On Fri 10-06-16 07:52:56, Nikola Pajkovsky wrote:
> >> Jan Kara <jack@...e.cz> writes:
> >> > On Thu 09-06-16 09:23:29, Nikola Pajkovsky wrote:
> >> >> Holger Hoffstätte <holger@...lied-asynchrony.com> writes:
> >> >> 
> >> >> > On Wed, 08 Jun 2016 14:56:31 +0200, Jan Kara wrote:
> >> >> > (snip)
> >> >> >> Attached patch fixes the issue for me. I'll submit it once a full xfstests
> >> >> >> run finishes for it (which may take a while as our server room is currently
> >> >> >> moving to a different place).
> >> >> >> 
> >> >> >> 								Honza
> >> >> >> -- 
> >> >> >> Jan Kara <jack@...e.com>
> >> >> >> SUSE Labs, CR
> >> >> >> From 3a120841a5d9a6c42bf196389467e9e663cf1cf8 Mon Sep 17 00:00:00 2001
> >> >> >> From: Jan Kara <jack@...e.cz>
> >> >> >> Date: Wed, 8 Jun 2016 10:01:45 +0200
> >> >> >> Subject: [PATCH] ext4: Fix deadlock during page writeback
> >> >> >> 
> >> >> >> Commit 06bd3c36a733 (ext4: fix data exposure after a crash) uncovered a
> >> >> >> deadlock in ext4_writepages() which was previously much harder to hit.
> >> >> >> After this commit xfstest generic/130 reproduces the deadlock on small
> >> >> >> filesystems.
> >> >> >
> >> >> > Since you marked this for -stable, just a heads-up that the previous patch
> >> >> > for the data exposure was rejected from -stable (see [1]) because it
> >> >> > has the mismatching "!IS_NOQUOTA(inode) &&" line, which didn't exist
> >> >> > until 4.6. I removed it locally but Greg probably wants an official patch.
> >> >> >
> >> >> > So both this and the previous patch need to be submitted.
> >> >> >
> >> >> > [1] http://permalink.gmane.org/gmane.linux.kernel.stable/18074{4,5,6}
> >> >> 
> >> >> I'm just wondering if the Jan's patch is not related to blocked
> >> >> processes in following trace. It very hard to hit it and I don't have
> >> >> any reproducer.
> >> >
> >> > This looks like a different issue. Does the machine recover itself or is it
> >> > a hard hang and you have to press a reset button?
> >> 
> >> The machine is bit bigger than I have pretend. It's 18 vcpu with 160 GB
> >> ram and machine has dedicated mount point only for PostgreSQL data.
> >> 
> >> Nevertheless, I was able always to ssh to the machine, so machine itself
> >> was not in hard hang and ext4 mostly gets recover by itself (it took
> >> 30min). But I have seen situation, were every process who 'touch' the ext4
> >> goes immediately to D state and does not recover even after hour.
> >
> > If such situation happens, can you run 'echo w >/proc/sysrq-trigger' to
> > dump stuck processes and also run 'iostat -x 1' for a while to see how much
> > IO is happening in the system? That should tell us more.
> 
> 
> Link to 'echo w >/proc/sysrq-trigger' is here, because it's bit bigger
> to mail it.
> 
>    http://expirebox.com/download/68c26e396feb8c9abb0485f857ccea3a.html

Can you upload it again please? I've got to looking at the file only today
and it is already deleted. Thanks!

> I was running iotop and there was traffic roughly ~20 KB/s write.
> 
> What was bit more interesting, was looking at
> 
>    cat /proc/vmstat | egrep "nr_dirty|nr_writeback"
> 
> nr_drity had around 240 and was slowly counting up, but nr_writeback had
> ~8800 and was stuck for 120s.

Hum, interesting. This would suggest like IO completion got stuck for some
reason. We'll see more from the stacktraces hopefully.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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