Re: [PATCH 04/10] xfs: implement freezing by emptying the AIL

Mark Tinguely
2012-04-18T13:13:58Z

On 04/17/12 03:26, Dave Chinner wrote:
> On Tue, Apr 17, 2012 at 02:20:23PM +1000, Dave Chinner wrote:
>> On Tue, Apr 17, 2012 at 09:54:32AM +1000, Dave Chinner wrote:
>>> On Mon, Apr 16, 2012 at 08:47:00AM -0500, Mark Tinguely wrote:
>>>> On 03/27/12 11:44, Christoph Hellwig wrote:
>>>>> Now that we write back all metadata either synchronously or through the AIL
>>>>> we can simply implement metadata freezing in terms of emptying the AIL.
>>>>>
>>>>> The implementation for this is fairly simply and straight-forward: A new
>>>>> routine is added that increments a counter that tells xfsaild to not stop
>>>>> until the AIL is empty and then waits on a wakeup from
>>>>> xfs_trans_ail_delete_bulk to signal that the AIL is empty.
>>>>>
>>>>> As usual the devil is in the details, in this case the filesystem shutdown
>>>>> code. Currently we are a bit sloppy there and do not continue ail pushing
>>>>> in that case, and thus never reach the code in the log item implementations
>>>>> that can unwind in case of a shutdown filesystem. Also the code to
>>>>> abort inode and dquot flushes was rather sloppy before and did not remove
>>>>> the log items from the AIL, which had to be fixed as well.
>>>>>
>>>>> Also treat unmount the same way as freeze now, except that we still keep a
>>>>> synchronous inode reclaim pass to make sure we reclaim all clean inodes, too.
>>>>>
>>>>> As an upside we can now remove the radix tree based inode writeback and
>>>>> xfs_unmountfs_writesb.
>>>>>
>>>>> Signed-off-by: Christoph Hellwig<hch@lst.de>
>>>>
>>>> Sorry for the empty email.
>>>>
>>>> This series hangs my test boxes. This patch is the first indication
>>>> of the hang. Reboot, and remove patch 4 and the test are successful.
>>>>
>>>> The machine is still responsive. Only the SCRATCH filesystem from
>>>> the test suite is hung.
>>>>
>>>> Per Dave's observation, I added a couple inode reclaims to this
>>>> patch and the test gets further (hangs on run 9 of test 068 rather
>>>> than run 3).
>>>
>>> That implies that there are dirty inodes at the VFS level leaking
>>> through the freeze.
>>>
>>> .....
>> .....
>>> So, what are the flusher threads doing - where are they stuck?
>>
>> I have an answer of sorts:
>>
>> 90580.054767] task PC stack pid father
>> [90580.056035] flush-253:16 D 0000000000000001 4136 32084 2 0x00000000
>> [90580.056035] ffff880004c558a0 0000000000000046 ffff880068b6cd48 ffff880004c55cb0
>> [90580.056035] ffff88007b616280 ffff880004c55fd8 ffff880004c55fd8 ffff880004c55fd8
>> [90580.056035] ffff88000681e340 ffff88007b616280 ffff880004c558b0 ffff88007981e000
>> [90580.056035] Call Trace:
>> [90580.056035] [<ffffffff81afcd19>] schedule+0x29/0x70
>> [90580.056035] [<ffffffff814801fd>] xfs_trans_alloc+0x5d/0xb0
>> [90580.056035] [<ffffffff81099eb0>] ? add_wait_queue+0x60/0x60
>> [90580.056035] [<ffffffff81416b14>] xfs_setfilesize_trans_alloc+0x34/0xb0
>> [90580.056035] [<ffffffff814186f5>] xfs_vm_writepage+0x4a5/0x560
>> [90580.056035] [<ffffffff81127507>] __writepage+0x17/0x40
>> [90580.056035] [<ffffffff81127b3d>] write_cache_pages+0x20d/0x460
>> [90580.056035] [<ffffffff811274f0>] ? set_page_dirty_lock+0x60/0x60
>> [90580.056035] [<ffffffff81127dda>] generic_writepages+0x4a/0x70
>> [90580.056035] [<ffffffff814167ec>] xfs_vm_writepages+0x4c/0x60
>> [90580.056035] [<ffffffff81129711>] do_writepages+0x21/0x40
>> [90580.056035] [<ffffffff8118ee42>] writeback_single_inode+0x112/0x380
>> [90580.056035] [<ffffffff8118f25e>] writeback_sb_inodes+0x1ae/0x270
>> [90580.056035] [<ffffffff8118f4c0>] wb_writeback+0xe0/0x320
>> [90580.056035] [<ffffffff8108724a>] ? try_to_del_timer_sync+0x8a/0x110
>> [90580.056035] [<ffffffff81190bc8>] wb_do_writeback+0xb8/0x1d0
>> [90580.056035] [<ffffffff81085f40>] ? usleep_range+0x50/0x50
>> [90580.056035] [<ffffffff81190d6b>] bdi_writeback_thread+0x8b/0x280
>> [90580.056035] [<ffffffff81190ce0>] ? wb_do_writeback+0x1d0/0x1d0
>> [90580.056035] [<ffffffff81099403>] kthread+0x93/0xa0
>> [90580.056035] [<ffffffff81b06f64>] kernel_thread_helper+0x4/0x10
>> [90580.056035] [<ffffffff81099370>] ? kthread_freezable_should_stop+0x70/0x70
>> [90580.056035] [<ffffffff81b06f60>] ? gs_change+0x13/0x13
>>
>> A dirty inode has slipped through the freeze process, and the
>> flusher thread is stuck trying to allocate a transaction for setting
>> the file size. I can reproduce this fairly easily, so a a bit of
>> tracing should tell me exactly what is going wrong....
>
> Yeah, it's pretty clear what is happening here. We don't have
> freeze protection against EOF zeroing operations. At least
> xfs_setattr_size() and xfs_change_file_space() fail to check for
> freeze, and that is initially what I though was causing this problem.
>
> However, adding freeze checks into the relevant paths didn't make
> the hangs go away, so there's more to it than that. Basically, we've
> been getting races between checking for freeze, the dirtying of the
> pages and the flusher thread syncing out the dirty data. i.e.:
>
> Thread 1 Thread 2 freeze flusher thread
> write inode A
> check for freeze
> grab s_umount
> SB_FREEZE_WRITE
> writeback_inodes_sb()
> iterate dirty inodes
> inode A not in flush
> sync_inodes_sb()
> iterate dirty inodes
> inode A not in flush
> dirty pages
> mark inode A dirty
> write inode A done.
> SB_FREEZE_TRANS
> drop s_umount
> freeze done
> sync
> grab s_umount
> iterate dirty inodes
> Flush dirty inode A
>
>
> Before we added the transactional inode size updates, this race
> simply went unnoticed because nothing caused the flusher thread to
> block. All the problems I see are due to overwrites of allocated
> space - if there was real allocation then the delalloc conversion
> would have always hung. Now we see that when we need to extend the
> file size when writing, we ahve to allocate a transaction and hence
> the flusher thread now hangs.
>
> While I can "fix" the xfs_setattr_size() and xfs_change_file_space()
> triggers, they don't close the above race condition, so this problem
> is essentially unfixable in XFS. The only reason we have not tripped
> over it before is that the flusher thread didn't hang waiting for a
> transaction reservation when the race was hit.
>
> So why didn't this happen before Christoph's patch set? That's
> something I can't explain. Oh, wait, yes I can - 068 hangs even
> without this patch of Christoph's. Actually, looking at my xfstests
> logs, I can trace the start of the failures back to mid march, and
> that coincided with an update to the xfstests installed on my test
> boxes. Which coincides with when my machines first saw this change:
>
> commit 281627df3eb55e1b729b9bb06fff5ff112929646
> Author: Christoph Hellwig<hch@infradead.org>
> Date: Tue Mar 13 08:41:05 2012 +0000
>
> xfs: log file size updates at I/O completion time
>
> That confirms my analysis above - the problem is being exposed by new
> code in the writeback path that does transaction allocation where it
> didn't used to.
>
> Clearly the problem is not really the new code in Christoph's
> patches - it's an existing freeze problem that has previously
> resulted in data writes occuring after a freeze has completed (of
> which we have had rare complaints about). That sounds pretty dire,
> except for one thing: Jan Kara's patch set that fixes all these
> freeze problems:
>
> https://lkml.org/lkml/2012/4/16/356
>
> And now that I've run some testing with Jan's patch series, along
> with Christoph's and mine (75-odd patches ;), a couple of my test
> VMs have been running test 068 in a tight loop for about half an
> hour without a hang, so I'd consider this problem fixed by Jan's
> freeze fixes given I could reliably hang it in 2-3 minutes before
> adding Jan's patch set to my stack.
>
> So the fix for this problem is getting Jan's patch set into the
> kernel at the same time we get the inode size logging changes into
> the kernel. What do people think about that for a plan?
>
> Cheers,
>
> Dave.

Good job.

Jan's freeze patch set is at v5 and seems to be settling down. What is
the status of Jan's freeze code getting into the kernel?

--Mark Tinguely