linux-kernel - Re: [PATCH] Fix mapping->writeback_index to point to the last written page

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Date:	Thu, 3 Mar 2011 15:08:02 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jun'ichi Nomura <j-nomura@...jp.nec.com>, Jan Kara <jack@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Fix mapping->writeback_index to point to the last
 written page

On Thu 03-03-11 21:31:19, Wu Fengguang wrote:
> On Thu, Mar 03, 2011 at 10:26:19AM +0800, Jun'ichi Nomura wrote:
>         >        flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899962 + 8
>         >        flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899970 + 8
>         >        flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899978 + 8
>         >        flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899986 + 8
>         >        flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899994 + 8
> ==>     >      kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94899962 + 40
>         > >>     flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898554 + 8
> ==>     > >>     flush-8:0-2743  4606: block_rq_issue: 8,0 W 0 () 94898554 + 8
> 
> I'd expect the wrapped around 94898554+8 to be merged with 94899962+8.
  How could they be merged? They are not continguous...

> Why kworker/0:1-11 is submitting the request early? And the second
> request is submitted by flush-8:0-2743.
  I'm not sure about that either - but I think that kworker is submitting
the work when unplug happens while flush ends up doing it when the queue is
alredy unplugged.
  
> > The 1st writeback ended at block 94898562. (94898554+8)
> > The 2nd writeback started there.
> > However, since the last page at the 1st writeback was just redirtied,
> > the 2nd writeback looped back to block 94898554 after sequentially
> > submitting blocks from 94898562 to 94900001.
> > 
> > 1 extra seek which could be avoided.
> > I haven't seen fatal problem with the latest kernel, though.
> > 
> > With older kernels (before 2.6.29, without commit 31a12666),
> > kupdate leaves the dirty pages like spots until the application wraps
> > around the ring. (It could take hours to days.)
> > That led me to this code.
> > 
> > > But as I'm thinking about it, it wouldn't harm our original aim to do
> > > what you propose and it can help this relatively common case. So I think
> > > it's a good idea. Fengguang, what do you think?
> 
> I see no problem too.
> 
> Tested-by: Wu Fengguang <fengguang.wu@...el.com>
  OK, I'll update the changelog to reflect our discussion and post the
patch to Andrew for inclusion.

								Honza
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