Re: [BUG] xfs/305 hangs 4.10-rc4 kernel

[Brian Foster <bfoster@redhat.com>]

On Fri, Jan 27, 2017 at 10:52:19AM +0800, Eryu Guan wrote:
> On Thu, Jan 26, 2017 at 01:44:28PM -0500, Brian Foster wrote:
> > On Thu, Jan 26, 2017 at 11:29:50AM +0800, Eryu Guan wrote:
> > > On Wed, Jan 25, 2017 at 09:52:16AM -0500, Brian Foster wrote:
> > > > On Wed, Jan 25, 2017 at 02:39:43PM +0800, Eryu Guan wrote:
> > > > > Hi all,
> > > > > 
> > > > > I hit another test hang triggered by running xfs/305 in a loop, it
> > > > > usually reproduces within 50 iterations. It also involves disabling
> > > > > quota, which looks similar to the generic/232 hang[1]. Perhaps they
> > > > > share the same root cause?
> > > > > 
> > > > > [333853.198159] xfs_quota       D    0 13880  13441 0x00000080
> > > > > [333853.227479] Call Trace:
> > > > > [333853.239429]  __schedule+0x21c/0x6b0
> > > > > [333853.255410]  schedule+0x36/0x80
> > > > > [333853.269932]  schedule_timeout+0x1d1/0x3a0
> > > > > [333853.288354]  ? lock_timer_base+0xa0/0xa0
> > > > > [333853.306296]  ? xfs_qm_need_dqattach+0x80/0x80 [xfs]
> > > > > [333853.329015]  schedule_timeout_uninterruptible+0x1f/0x30
> > > > > [333853.353344]  xfs_qm_dquot_walk.isra.10+0x172/0x190 [xfs]
> > > > > [333853.377747]  xfs_qm_dqpurge_all+0x5c/0x80 [xfs]
> > > > > [333853.398433]  xfs_qm_scall_quotaoff+0x127/0x380 [xfs]
> > > > > [333853.421062]  xfs_quota_disable+0x3d/0x50 [xfs]
> > > > > [333853.441319]  SyS_quotactl+0x391/0x850
> > > > > [333853.458143]  ? __audit_syscall_entry+0xaf/0x100
> > > > > [333853.478903]  ? syscall_trace_enter+0x1d0/0x2b0
> > > > > [333853.499262]  ? __audit_syscall_exit+0x209/0x290
> > > > > [333853.519938]  do_syscall_64+0x67/0x180
> > > > > [333853.536771]  entry_SYSCALL64_slow_path+0x25/0x25
> > > > > [333853.558105] RIP: 0033:0x7f95be064fea
> > > > > [333853.574521] RSP: 002b:00007ffc988b6348 EFLAGS: 00000206 ORIG_RAX: 00000000000000b3
> > > > > [333853.608922] RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007f95be064fea
> > > > > [333853.641478] RDX: 0000000000000000 RSI: 00000000027bb2f0 RDI: 0000000000580201
> > > > > [333853.674091] RBP: 00000000027bb320 R08: 00007ffc988b635c R09: 0000000000000001
> > > > > [333853.706913] R10: 00007ffc988b635c R11: 0000000000000206 R12: 00000000027bb2d0
> > > > > [333853.744953] R13: 00000000027ba0a0 R14: 0000000000000000 R15: 00000000027ba080
> > > > > 
> > > > > I've seen this hang on multiple test hosts with different test configs,
> > > > > e.g. xfs_2k_crc xfs_4k_reflink. I also tried with 4.9 kernel, but I kept
> > > > > hitting the BUG_ON that this kernel commit[2] fixed in 4.10-rc kernel.
> > > > > 
> > > > 
> > > > Interesting... I'm not sure this is necessarily locked up given the call
> > > > to schedule_timeout(). That suggests we could be in the
> > > > xfs_qm_dquot_walk()->delay() call and possibly spinning on skipped
> > > > entries. It looks like the only way we skip an entry is if it is already
> > > > being freed or has a reference count. The only other dquot free path
> > > > looks like the shrinker, but that doesn't appear in the blocked task
> > > > list at all. We do have a dqread call, but that path hasn't even added
> > > > the dquot to the radix tree yet.
> > > > 
> > > > > I attached full sysrq-w output. If you need more info please let me
> > > > > know.
> > > > >
> > > > 
> > > > It looks like pretty much everything is hung up on log reservation,
> > > > including writeback, which is more indicative of a potential log
> > > > reservation problem. xfsaild appears to be alive, however (a
> > > > schedule_timeout() in that path suggests the thread is working to push
> > > > log items and free up log space), so maybe something else is going on.
> > > > It might be interesting to see if we can tell if we're spinning
> > > > somewhere by enabling tracepoints once in this state. Also, have you
> > > > checked for any unexpected syslog messages when this occurs?
> > > 
> > > There's nothing interesting in dmesg with stock 4.10-rc4 kernel, but if
> > > I test with Darrick's xfs-linux tree for-next branch (which has a few
> > > patches on top of 4.10-rc4), I can see this message:
> > > 
> > > XFS (dm-5): xlog_verify_grant_tail: space > BBTOB(tail_blocks)
> > > 
> > > But I can see this message in every xfs/305 iteration, and other tests
> > > too. I attached dmesg log prior to sysrq-w output, gathered from
> > > "for-next" kernel.
> > > 
> > > I also attached trace log while we're in this state (also from
> > > "for-next" kernel):
> > > 
> > > cd /mnt/ext4
> > > trace-cmd record -e xfs_*
> > > sleep 10
> > > trace-cmd report > log
> > > 
> > 
> > Thanks Eryu. The only thing in the AIL is the XFS_LI_QUOTAOFF log item.
> > This item is hardcoded to the locked state because it does not really
> > exist on disk (i.e., it cannot be pushed). Rather, it looks like it is
> > released upon commit of an accompanying quotaoff_end log item that is
> > committed after most of the quotaoff work is completed, including the
> > purge that you're actually stuck on. The purpose of this appears to be
> > to allow log recovery to deal with the possibility of dquots being
> > logged after the initial quotaoff transaction. This mechanism
> > effectively pins the tail of the log with the quotaoff start log item to
> > ensure log recovery sees it before any subsequent dquot modifications if
> > we happen to crash at some point in the middle of those operations.
> > 
> > What this means is that there is only a fixed amount of physical log
> > space between the time the quotaoff start is logged and until the
> > quotaoff completes. If we consume the log space before the quotaoff
> > completes, we basically deadlock the fs. Indeed, while I haven't been
> > able to reproduce via xfs/305 alone, I can manufacture this problem by
> > inserting a long enough sleep after the qutoaoff start item is logged.
> > The parallel fsstress' then chew up the log and the fs ends up stuck.
> > 
> > So I think this is the state you get into, but the root cause is still
> > not totally clear. It could just be the workload, but it seems a little
> > crazy to wrap the log before the purge completes since it doesn't log
> > anything itself. Then again, it looks like the xfs/305 workload can chew
> > up the log in 30-45s on my 10g scratch dev, so probably not out of the
> > realm of possibility.
> > 
> > I am wondering about the possibility of something else getting stuck
> > that stalls the dqpurge and then causes the fs-wide deadlock via domino
> > effect. Could you provide the xfs_info of your scratch device? Also, if
> 
> # xfs_info /mnt/testarea/scratch/
> meta-data=/dev/mapper/systemvg-testlv2 isize=512    agcount=16, agsize=245696 blks
>          =                       sectsz=512   attr=2, projid32bit=1
>          =                       crc=1        finobt=1 spinodes=0 rmapbt=1
>          =                       reflink=1
> data     =                       bsize=4096   blocks=3931136, imaxpct=25
>          =                       sunit=64     swidth=192 blks
> naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
> log      =internal               bsize=4096   blocks=7680, version=2
>          =                       sectsz=512   sunit=64 blks, lazy-count=1
> realtime =none                   extsz=4096   blocks=0, rtextents=0
> 
> > you could make a system available that is able to reproduce this
> > reliably enough, that might make it easier to analyze what's going on...
> 
> I do have a system that could reproduce the hang reliably, but not so
> efficiently, it usually reproduces around 30 minutes by running xfs/305
> in a loop. It's a RH internal test host, I can provide you the address
> and password if you think it's useful.
> 

Thanks for the system info. I could reproduce fairly quickly. I adopted
some of the characteristics of your scratch dev on my local machine and
was able to reproduce there as well. What is of note is that in both
cases I reproduce with xfs_quota stuck at a different point from your
original report. Instead of being stuck in xfs_qm_dquot_walk(), I
reproduce the following stack:

[190961.556198] xfs_quota       D    0  9394   8970 0x00000080
[190961.581188] Call Trace:
[190961.592501]  __schedule+0x21c/0x6b0
[190961.608704]  ? radix_tree_gang_lookup+0xc8/0x110
[190961.629892]  schedule+0x36/0x80
[190961.644336]  xlog_grant_head_wait+0xd2/0x210 [xfs]
[190961.666212]  xlog_grant_head_check+0xaf/0x160 [xfs]
[190961.688687]  xfs_log_reserve+0xed/0x270 [xfs]
[190961.708753]  ? kmem_cache_alloc+0x194/0x1a0
[190961.727905]  xfs_trans_reserve+0x1d6/0x2c0 [xfs]
[190961.748958]  xfs_trans_alloc+0xc1/0x140 [xfs]
[190961.768899]  xfs_qm_log_quotaoff_end+0x3c/0x90 [xfs]
[190961.793068]  xfs_qm_scall_quotaoff+0x128/0x3b0 [xfs]
[190961.819012]  xfs_quota_disable+0x3d/0x50 [xfs]
[190961.839279]  SyS_quotactl+0x391/0x850
[190961.856210]  ? __audit_syscall_entry+0xaf/0x100
[190961.876958]  ? syscall_trace_enter+0x1d0/0x2b0
[190961.897212]  ? __audit_syscall_exit+0x209/0x290
[190961.917889]  do_syscall_64+0x67/0x180
[190961.934687]  entry_SYSCALL64_slow_path+0x25/0x25

... which suggests that we probably need to allocate the quotaoff_end
transaction right when we allocate the quotaoff start transaction and
stash it for use once we've released/purged all of the dquots.

There may be multiple issues here so I'll give something like that a
shot and see if I can still reproduce a hang.

Brian

> Thanks,
> Eryu
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