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 14:59:57 +0200
From:	Nikola Pajkovsky <nikola.pajkovsky@...ddata.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

Jan Kara <jack@...e.cz> writes:

> 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!

http://expirebox.com/download/c010e712e55938435c446cdc01a0b523.html

>> 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.

I have monitor /sys/kernel/debug/bdi/253:32/stats for 10 mins per 1 sec.
Values are all same as follows:

    --[ Sun Jun 19 06:11:08 CEST 2016
    BdiWriteback:            15840 kB
    BdiReclaimable:          32320 kB
    BdiDirtyThresh:              0 kB
    DirtyThresh:           1048576 kB
    BackgroundThresh:       131072 kB
    BdiDirtied:         6131163680 kB
    BdiWritten:         6130214880 kB
    BdiWriteBandwidth:      324948 kBps
    b_dirty:                     2
    b_io:                        3
    b_more_io:                   0
    bdi_list:                    1
    state:                       c

Maybe those values can cause issue and kicks in writeback to often and
block everyone else.

   $ sysctl -a | grep dirty | grep -v ratio
   vm.dirty_background_bytes = 134217728
   vm.dirty_bytes = 1073741824
   vm.dirty_expire_centisecs = 1500
   vm.dirty_writeback_centisecs = 500

I even have output of command, if you're interested.

  $ trace-cmd record -e ext4 -e jbd2 -e writeback -e block sleep 600

-- 
Nikola
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