linux-ext4 - Re: process hangs in ext4_sync

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Date:	Tue, 29 Oct 2013 11:06:41 +0530
From:	Sandeep Joshi <sanjos100@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: process hangs in ext4_sync_file

On Wed, Oct 23, 2013 at 8:28 PM, Sandeep Joshi <sanjos100@...il.com> wrote:
> On Wed, Oct 23, 2013 at 3:50 PM, Jan Kara <jack@...e.cz> wrote:
>> On Mon 21-10-13 18:09:02, Sandeep Joshi wrote:
>>> I am seeing a problem reported 4 years earlier
>>> https://lkml.org/lkml/2009/3/12/226
>>> (same stack as seen by Alexander)
>>>
>>> The problem is reproducible.  Let me know if you need any info in
>>> addition to that seen below.
>>>
>>> I have multiple threads in a process doing heavy IO on a ext4
>>> filesystem mounted with (discard, noatime) on a SSD or HDD.
>>>
>>> This is on Linux 3.8.0-29-generic #42~precise1-Ubuntu SMP Wed Aug 14
>>> 16:19:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
>>>
>>> For upto minutes at a time, one of the threads seems to hang in sync to disk.
>>>
>>> When I check the thread stack in /proc, I find that the stack is one
>>> of the following two
>>>
>>> <ffffffff81134a4e>] sleep_on_page+0xe/0x20
>>> [<ffffffff81134c88>] wait_on_page_bit+0x78/0x80
>>> [<ffffffff81134d9c>] filemap_fdatawait_range+0x10c/0x1a0
>>> [<ffffffff811367d8>] filemap_write_and_wait_range+0x68/0x80
>>> [<ffffffff81236a4f>] ext4_sync_file+0x6f/0x2b0
>>> [<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
>>> [<ffffffff81168fb3>] sys_msync+0x143/0x1d0
>>> [<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>
>>>
>>> OR
>>>
>>>
>>> [<ffffffff812947f5>] jbd2_log_wait_commit+0xb5/0x130
>>> [<ffffffff81297213>] jbd2_complete_transaction+0x53/0x90
>>> [<ffffffff81236bcd>] ext4_sync_file+0x1ed/0x2b0
>>> [<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
>>> [<ffffffff81168fb3>] sys_msync+0x143/0x1d0
>>> [<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>
>>> Any clues?
>>   We are waiting for IO to complete. As the first thing, try to remount
>> your filesystem without 'discard' mount option. That is often causing
>> problems.
>>
>>                                                                 Honza

Update : I removed the "discard" option as Jan/Honza suggested and I
dont see processes hanging in ext4_sync_file anymore.   I also
replaced ext4 with ext2 and no problems there either.

So isn't the "discard' option recommended for SSDs?   Is this a known
problem with ext4?

-Sandeep

>
>
> Thanks Jan,  I will remove it and see what happens.
> I was also planning to switch to ext2 and see if the failure continues.
> I added the discard option because the filesystem was initially
> supposed to be on an SSD
>
> is there any document which tells me what to look for in the output of
> "echo w > /proc/sysrq-trigger" ?
>
> -Sandeep
>
>>
>> --
>> Jan Kara <jack@...e.cz>
>> SUSE Labs, CR
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