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From: Martin Svec <martin.svec@zoner.cz>
To: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>, linux-xfs@vger.kernel.org
Subject: Re: Quota-enabled XFS hangs during mount
Date: Fri, 27 Jan 2017 14:06:45 +0100	[thread overview]
Message-ID: <27c2f5aa-517d-22b5-b55f-f0ceb277e9a3@zoner.cz> (raw)
In-Reply-To: <20170126191254.GB39683@bfoster.bfoster>

Dne 26.1.2017 v 20:12 Brian Foster napsal(a):
> On Thu, Jan 26, 2017 at 06:46:42PM +0100, Martin Svec wrote:
>> Hello,
>>
>> Dne 25.1.2017 v 23:17 Brian Foster napsal(a):
>>> On Tue, Jan 24, 2017 at 02:17:36PM +0100, Martin Svec wrote:
>>>> Hello,
>>>>
>>>> Dne 23.1.2017 v 14:44 Brian Foster napsal(a):
>>>>> On Mon, Jan 23, 2017 at 10:44:20AM +0100, Martin Svec wrote:
>>>>>> Hello Dave,
>>>>>>
>>>>>> Any updates on this? It's a bit annoying to workaround the bug by increasing RAM just because of the
>>>>>> initial quotacheck.
>>>>>>
>>>>> Note that Dave is away on a bit of an extended vacation[1]. It looks
>>>>> like he was in the process of fishing through the code to spot any
>>>>> potential problems related to quotacheck+reclaim. I see you've cc'd him
>>>>> directly so we'll see if we get a response wrt to if he got anywhere
>>>>> with that...
>>>>>
>>>>> Skimming back through this thread, it looks like we have an issue where
>>>>> quota check is not quite reliable in the event of reclaim, and you
>>>>> appear to be reproducing this due to a probably unique combination of
>>>>> large inode count and low memory.
>>>>>
>>>>> Is my understanding correct that you've reproduced this on more recent
>>>>> kernels than the original report? 
>>>> Yes, I repeated the tests using 4.9.3 kernel on another VM where we hit this issue.
>>>>
>>>> Configuration:
>>>> * vSphere 5.5 virtual machine, 2 vCPUs, virtual disks residing on iSCSI VMFS datastore
>>>> * Debian Jessie 64 bit webserver, vanilla kernel 4.9.3
>>>> * 180 GB XFS data disk mounted as /www
>>>>
>>>> Quotacheck behavior depends on assigned RAM:
>>>> * 2 or less GiB: mount /www leads to a storm of OOM kills including shell, ttys etc., so the system
>>>> becomes unusable.
>>>> * 3 GiB: mount /www task hangs in the same way as I reported in earlier in this thread.
>>>> * 4 or more GiB: mount /www succeeds.
>>>>
>>> I was able to reproduce the quotacheck OOM situation on latest kernels.
>>> This problem actually looks like a regression as of commit 17c12bcd3
>>> ("xfs: when replaying bmap operations, don't let unlinked inodes get
>>> reaped"), but I don't think that patch is the core problem. That patch
>>> pulled up setting MS_ACTIVE on the superblock from after XFS runs
>>> quotacheck to before it (for other reasons), which has a side effect of
>>> causing inodes to be placed onto the lru once they are released. Before
>>> this change, all inodes were immediately marked for reclaim once
>>> released from quotacheck because the superblock had not been set active.
>>>
>>> The problem here is first that quotacheck issues a bulkstat and thus
>>> grabs and releases every inode in the fs. The quotacheck occurs at mount
>>> time, which means we still hold the s_umount lock and thus the shrinker
>>> cannot run even though it is registered. Therefore, we basically just
>>> populate the lru until we've consumed too much memory and blow up.
>>>
>>> I think the solution here is to preserve the quotacheck behavior prior
>>> to commit 17c12bcd3 via something like the following:
>>>
>>> --- a/fs/xfs/xfs_qm.c
>>> +++ b/fs/xfs/xfs_qm.c
>>> @@ -1177,7 +1177,7 @@ xfs_qm_dqusage_adjust(
>>>  	 * the case in all other instances. It's OK that we do this because
>>>  	 * quotacheck is done only at mount time.
>>>  	 */
>>> -	error = xfs_iget(mp, NULL, ino, 0, XFS_ILOCK_EXCL, &ip);
>>> +	error = xfs_iget(mp, NULL, ino, XFS_IGET_DONTCACHE, XFS_ILOCK_EXCL, &ip);
>>>  	if (error) {
>>>  		*res = BULKSTAT_RV_NOTHING;
>>>  		return error;
>>>
>>> ... which allows quotacheck to run as normal in my quick tests. Could
>>> you try this on your more recent kernel tests and see whether you still
>>> reproduce any problems?
>> The above patch fixes OOM issues and reduces overall memory consumption during quotacheck. However,
>> it does not fix the original xfs_qm_flush_one() freezing. I'm still able to reproduce it with 1 GB
>> of RAM or lower. Tested with 4.9.5 kernel.
>>
> Ok, thanks. I'll get that fix posted shortly.
>
> I hadn't tried reducing RAM any further. I dropped my vm down to 1GB and
> I don't reproduce a hang. If I drop to 512MB, the mount actually crashes
> due to what looks like the problem that djwong just fixed[1].
>
> With that one liner applied, it does look like I've hit a mount hang in
> the quotacheck path. Note that I'm also running into OOM issues again
> though, probably due to legitimately not having enough RAM for this vm.
> Anyways, I'll see if I can dig anything out of that...
>
> FWIW, this is all on the latest for-next (4.10.0-rc5).
>
> [1] http://www.spinics.net/lists/linux-xfs/msg03869.html
>
>> If it makes sense to you, I can rsync the whole filesystem to a new XFS volume and repeat the tests.
>> At least, that could tell us if the problem depends on a particular state of on-disk metadata
>> structures or it's a general property of the given filesystem tree.
>>
> That couldn't hurt, thanks.
>

Well, after rsync to a fresh non-resized XFS volume, I still hit the mount hang with 1GB RAM.

Martin


  reply	other threads:[~2017-01-27 13:06 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-01 16:45 Martin Svec
2016-11-01 21:58 ` Dave Chinner
2016-11-02 16:31   ` Martin Svec
2016-11-03  1:31     ` Dave Chinner
2016-11-03 12:04       ` Martin Svec
2016-11-03 20:40         ` Dave Chinner
2017-01-23  9:44           ` Martin Svec
2017-01-23 13:44             ` Brian Foster
2017-01-23 22:06               ` Dave Chinner
2017-01-24 13:17               ` Martin Svec
2017-01-25 15:36                 ` Brian Foster
2017-01-25 22:17                 ` Brian Foster
2017-01-26 17:46                   ` Martin Svec
2017-01-26 19:12                     ` Brian Foster
2017-01-27 13:06                       ` Martin Svec [this message]
2017-01-27 17:07                         ` Brian Foster
2017-01-27 20:49                           ` Martin Svec
2017-01-27 21:00                             ` Martin Svec
2017-01-27 23:17                               ` Darrick J. Wong
2017-01-28 22:42                           ` Dave Chinner
2017-01-30 15:31                             ` Brian Foster

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