From: Chris Friesen <chris.friesen@windriver.com>
Subject: Re: RT/ext4/jbd2 circular dependency
Date: Wed, 29 Oct 2014 14:17:53 -0600
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Cc: Austin Schuh <austin@peloton-tech.com>, <pavel@pavlinux.ru>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	<linux-ext4@vger.kernel.org>, <tytso@mit.edu>,
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To: Thomas Gleixner <tglx@linutronix.de>
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On 10/29/2014 01:26 PM, Thomas Gleixner wrote:
> On Wed, 29 Oct 2014, Chris Friesen wrote:
>> On 10/29/2014 12:05 PM, Thomas Gleixner wrote:
>> It seems plausible that the reason why page writeback never completes is that
>> it's blocking trying to take inode->i_data_sem for reading, as seen in the
>> following stack trace (from a hung system):
>>
>> [<ffffffff8109cd0c>] rt_down_read+0x2c/0x40
>> [<ffffffff8120ac91>] ext4_map_blocks+0x41/0x270
>> [<ffffffff8120f0dc>] mpage_da_map_and_submit+0xac/0x4c0
>> [<ffffffff8120f9c9>] write_cache_pages_da+0x3f9/0x420
>> [<ffffffff8120fd30>] ext4_da_writepages+0x340/0x720
>> [<ffffffff8111a5f4>] do_writepages+0x24/0x40
>> [<ffffffff81191b71>] writeback_single_inode+0x181/0x4b0
>> [<ffffffff811922a2>] writeback_sb_inodes+0x1b2/0x290
>> [<ffffffff8119241e>] __writeback_inodes_wb+0x9e/0xd0
>> [<ffffffff811928e3>] wb_writeback+0x223/0x3f0
>> [<ffffffff81192b4f>] wb_check_old_data_flush+0x9f/0xb0
>> [<ffffffff8119403f>] wb_do_writeback+0x12f/0x250
>> [<ffffffff811941f4>] bdi_writeback_thread+0x94/0x320
>
> Well, the point is that the JBD write out is not completed. The above
> is just the consequence. So really looking at ext4 inode write backs
> and something stuck on BJ_Shadow or the inode sem is the wrong
> place. It's all just caused by the JDB writeout not being completed
> for whatever reason.

I'll willingly confess my ignorance of filesystem code before I started 
looking at this issue.  I was under the impression that the above stack 
trace (for the "flush-147:3" task, in this case) was performing the 
write out of the page that had been flagged for writeback by JBD...is 
that not the case?  If not, then could you point me in the right direction?

>> For what it's worth, I'm currently testing a backport of commit b34090e from
>> mainline (which in turn required backporting commits e5a120a and f5113ef).  It
>> switches from using the BJ_Shadow list to using the BH_Shadow flag on the
>> buffer head.  More interestingly, waiters now get woken up from
>> journal_end_buffer_io_sync() instead of from
>> jbd2_journal_commit_transaction().
>>
>> So far this seems to be helping a lot.  It's lasted about 15x as long under
>> stress as without the patches.
>
> I fear that this is just papering over the problem, but you have to
> talk to the jbd2 folks about that.

They're on the CC list, hopefully they'll chime in...

> I personally prefer a reasonable explanation for the current behaviour
> rather than a magic "solution" to the problem. But that's up to you.

Well sure...but if nothing else it helps to point to a possible cause. 
I'm currently looking at the locking implications of the patch to see if 
it completely closes the race window or just narrows it.

Chris