linux-ext4 - Re: DIO process stuck apparently due to dioread

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Date:	Thu, 11 Aug 2011 19:46:10 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: DIO process stuck apparently due to dioread_nolock (3.0)

Hi Michael,

Could you try your test with the patch I just posted:
http://marc.info/?l=linux-ext4&m=131311627101278&w=2
and see whether it fixes the problem?

Thanks!

Jiaying

On Thu, Aug 11, 2011 at 1:05 PM, Michael Tokarev <mjt@....msk.ru> wrote:
> 11.08.2011 18:01, Jan Kara wrote:
> []
>>>>> When it happens, a process doing direct AIO stalls
>>>>> infinitely, with the following backtrace:
>>>>>
>>>>> [87550.759848] INFO: task oracle:23176 blocked for more than 120 seconds.
>>>>> [87550.759892] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>>>> [87550.759955] oracle          D 0000000000000000     0 23176      1 0x00000000
>>>>> [87550.760006]  ffff8820457b47d0 0000000000000082 ffff880600000000 ffff881278e3f7d0
>>>>> [87550.760085]  ffff8806215c1fd8 ffff8806215c1fd8 ffff8806215c1fd8 ffff8820457b47d0
>>>>> [87550.760163]  ffffea0010bd7c68 ffffffff00000000 ffff882045512ef8 ffffffff810eeda2
>>>>> [87550.760245] Call Trace:
>>>>> [87550.760285]  [<ffffffff810eeda2>] ? __do_fault+0x422/0x520
>>>>> [87550.760327]  [<ffffffff81111ded>] ? kmem_getpages+0x5d/0x170
>>>>> [87550.760367]  [<ffffffff81112e58>] ? ____cache_alloc_node+0x48/0x140
>>>>> [87550.760430]  [<ffffffffa0123e6d>] ? ext4_file_write+0x20d/0x260 [ext4]
>>>>> [87550.760475]  [<ffffffff8106aee0>] ? abort_exclusive_wait+0xb0/0xb0
>>>>> [87550.760523]  [<ffffffffa0123c60>] ? ext4_llseek+0x120/0x120 [ext4]
>>>>> [87550.760566]  [<ffffffff81162173>] ? aio_rw_vect_retry+0x73/0x1d0
>>>>> [87550.760607]  [<ffffffff8116302f>] ? aio_run_iocb+0x5f/0x160
>>>>> [87550.760646]  [<ffffffff81164258>] ? do_io_submit+0x4f8/0x600
>>>>> [87550.760689]  [<ffffffff81359b52>] ? system_call_fastpath+0x16/0x1b
>>>>   Hmm, the stack trace does not quite make sense to me - the part between
>>>> __do_fault and aio_rw_vect_retry is somehow broken. I can imagine we
>>>> blocked in ext4_file_write() but I don't see any place there where we would
>>>> allocate memory. By any chance, are there messages like "Unaligned AIO/DIO
>>>> on inode ..." in the kernel log?
>>>
>>> Yes, there are warnings about unaligned DIO, referring to this same
>>> process actually. Oracle does almost good job at aligning writes
>>> (usually it does i/o by its blocks which are 4Kb by default but
>>> are set to something larger - like 16Kb - for larger database).
>>> Except of a few cases, and lgwr process is one of them (*) - it
>>> writes logfiles using 512b blocks.  This is okay for a raw device
>>> with 512bytes blocks, but ext4 expects 4k writes at min.
>>>
>>> (*) another case is writing to control file, which is also done in
>>> 512byte chunks.
>>   Ah, OK. I think the code ext4_end_io_nolock() handling the wait queue
>> might be racy. waitqueue_active() check is missing a barrier I think.
>> Does attached (untested) patch fix the issue for you?
>
> With this patch applied, I can reproduce the problem easily too:
>
> [   76.982985] EXT4-fs (dm-1): Unaligned AIO/DIO on inode 3407879 by oracle; performance will be poor.
> [ 1469.734114] SysRq : Show Blocked State
> [ 1469.734157]   task                        PC stack   pid father
> [ 1469.734473] oracle          D 0000000000000000     0  6146      1 0x00000000
> [ 1469.734525]  ffff88103f604810 0000000000000082 ffff881000000000 ffff881079791040
> [ 1469.734603]  ffff880432c19fd8 ffff880432c19fd8 ffff880432c19fd8 ffff88103f604810
> [ 1469.734681]  ffffea000ec13590 ffffffff00000000 ffff881438c8dad8 ffffffff810eeda2
> [ 1469.734760] Call Trace:
> [ 1469.734800]  [<ffffffff810eeda2>] ? __do_fault+0x422/0x520
> [ 1469.734863]  [<ffffffffa0123e6d>] ? ext4_file_write+0x20d/0x260 [ext4]
> [ 1469.734909]  [<ffffffff8106aee0>] ? abort_exclusive_wait+0xb0/0xb0
> [ 1469.734956]  [<ffffffffa0123c60>] ? ext4_llseek+0x120/0x120 [ext4]
> [ 1469.734999]  [<ffffffff81162173>] ? aio_rw_vect_retry+0x73/0x1d0
> [ 1469.735039]  [<ffffffff8116302f>] ? aio_run_iocb+0x5f/0x160
> [ 1469.735078]  [<ffffffff81164258>] ? do_io_submit+0x4f8/0x600
> [ 1469.735122]  [<ffffffff81359b52>] ? system_call_fastpath+0x16/0x1b
>
> which is exactly the same as with previous "w" into sysrq-trigger.
>
> So it's something else... :(
>
> Thank you!
>
> /mjt
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