Re: Quota-enabled XFS hangs during mount

[Martin Svec <martin.svec@zoner.cz>]

Dne 27.1.2017 v 18:07 Brian Foster napsal(a):
> On Fri, Jan 27, 2017 at 02:06:45PM +0100, Martin Svec wrote:
>> 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.
>>
> The problem looks like a race between dquot reclaim and quotacheck. The
> high level sequence of events is as follows:
>
>  - During quotacheck, xfs_qm_dqiterate() walks the physical dquot
>    buffers and queues them to the delwri queue.
>  - Next, kswapd kicks in and attempts to reclaim a dquot that is backed
>    by a buffer on the quotacheck delwri queue. xfs_qm_dquot_isolate()
>    acquires the flush lock and attempts to queue to the reclaim delwri
>    queue. This silently fails because the buffer is already queued.
>
>    From this point forward, the dquot flush lock is not going to be
>    released until the buffer is submitted for I/O and completed via
>    quotacheck.
>  - Quotacheck continues on to the xfs_qm_flush_one() pass, hits the
>    dquot in question and waits on the flush lock to issue the flush of
>    the recalculated values. *deadlock*
>
> There are at least a few ways to deal with this. We could do something
> granular to fix up the reclaim path to check whether the buffer is
> already queued or something of that nature before we actually invoke the
> flush. I think this is effectively pointless, however, because the first
> part of quotacheck walks and queues all physical dquot buffers anyways.
>
> In other words, I think dquot reclaim during quotacheck should probably
> be bypassed. Given that, we could either adjust when the shrinker is
> registered until after quotacheck or set a flag somewhere to cause dquot
> reclaim to back out when quotacheck is running. I opted for something
> like the latter. Care to test the appended patch?
>
> Note that I think this does mean that you could still have low memory
> issues if you happen to have a lot of quotas defined..
>

Looks good, no more hangs with 1 GB. Thank you, Brian.

If I further reduce RAM to 512 MB, mount succeeds too but multiple "BUG: Bad page state in process
mount" errors are reported. Is it one of the expected low memory issues?

Martin