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

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Date:	Sat, 12 Mar 2016 23:27:23 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	"HUANG Weller (CM/ESW12-CN)" <Weller.Huang@...bosch.com>,
	"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

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

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