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

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Date:	Mon, 14 Mar 2016 10:36:35 -0400
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 Mon, Mar 14, 2016 at 08:39:28AM +0100, Jan Kara wrote:
> No, that won't be enough. blkdev_issue_flush() is not guaranteed to do
> anything to IOs which have not reported completion before
> blkdev_issue_flush() was called. Specifically, CFQ will queue submitted bio
> in its internal RB tree, following flush request completely bypasses this
> tree and goes directly to the disk where it flushes caches. And only later
> CFQ decides to schedule async writeback from the flusher thread which is
> queued in the RB tree...

Oh, right.  I am forgetting about the flushing mahchinery rewrite.
Thanks for pointing that out.

But what we *could* do is to swap those two calls and then in the case
where delalloc is enabled, could maintain a list of inodes where we
only need to call filemap_fdatawait(), and not initiate writeback for
any dirty pages which had been caused by non-allocating writes.

						- Ted
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