linux-ext4 - RE: ext4 out of order when use cfq scheduler

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Message-ID: <ec4578ecc2c94f69b43f637028502073@SGPMBX1004.APAC.bosch.com>
Date:	Mon, 14 Mar 2016 02:43:03 +0000
From:	"HUANG Weller (CM/ESW12-CN)" <Weller.Huang@...bosch.com>
To:	Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Li, Michael" <huayil@....qualcomm.com>
Subject: RE: ext4 out of order when use cfq scheduler



> -----Original Message-----
> From: Theodore Ts'o [mailto:tytso@....edu]
> Sent: Sunday, March 13, 2016 12:27 PM
> To: Jan Kara <jack@...e.cz>
> Cc: HUANG Weller (CM/ESW12-CN) <Weller.Huang@...bosch.com>; linux-
> ext4@...r.kernel.org; Li, Michael <huayil@....qualcomm.com>
> Subject: Re: ext4 out of order when use cfq scheduler
> 
> On Thu, Jan 07, 2016 at 12:47:36PM +0100, Jan Kara wrote:
> >
> > The problem is in all kernels starting with 3.8. Attached is a patch
> > which should fix the issue. Can you test whether it fixes the problem for you?
> 
> Sorry, I missed this patch because it was attached to an discussion thread.
> 
> > The problem is that although for delayed allocated blocks we write
> > their contents immediately after allocating them, there is no
> > guarantee that the IO scheduler or device doesn't reorder things
> 
> I don't think that's the problem.  In the commit thread when we call
> blkdev_issue_flush() that acts as a barrier so the I/O scheduler won't reorder writes
> after that point, which is before we write the commit block.  Instead, I believe the
> problem is in ext4_writepages:
> 
> 		ext4_journal_stop(handle);
> 		/* Submit prepared bio */
> 		ext4_io_submit(&mpd.io_submit);
> 
> Once we release the handle, the commit can start --- *before* we have
> a chance to submit the I/O.   Oops.
> 
> I believe if we swap these two calls, it should fix the problem Huang was seeing.
> 
> Jan, do you agree?
> 
> 					- Ted

Hi Ted and Jan,

You can give me a patch and I can redo the verification on my kernel and HWs.

I also look into the code, since In my test case, I use data=ordered option and without sync.  So the write operation will goto ext4_da_writepages(), right ?
My kernel version is 3.10.63, as I see io_submit and journal_stop sequence  already in that order. 

while (!ret && wbc->nr_to_write > 0) {
ext4_journal_start
write_cache_pages_da
mpage_da_map_and_submit  ==>
ext4_journal_stop
}

Thanks.
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