linux-kernel - Re: Bug in kernel 2.6.31, Slow wb

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Message-ID: <20090730014917.GB7326@localhost>
Date:	Thu, 30 Jul 2009 09:49:17 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Martin Bligh <mbligh@...gle.com>
Cc:	Chad Talbott <ctalbott@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Michael Rubin <mrubin@...gle.com>,
	Andrew Morton <akpm@...gle.com>,
	"sandeen@...hat.com" <sandeen@...hat.com>,
	Michael Davidson <md@...gle.com>
Subject: Re: Bug in kernel 2.6.31, Slow wb_kupdate writeout

On Thu, Jul 30, 2009 at 08:19:34AM +0800, Martin Bligh wrote:
> BTW, can you explain this code at the bottom of generic_sync_sb_inodes
> for me?
> 
>                 if (wbc->nr_to_write <= 0) {
>                         wbc->more_io = 1;
>                         break;
>                 }
> 
> I don't understand why we are setting more_io here? AFAICS, more_io
> means there's more stuff to write ... I would think we'd set this if
> nr_to_write was > 0 ?

That's true: wbc.nr_to_write will always be set to MAX_WRITEBACK_PAGES
by wb_writeback() before entering generic_sync_sb_inodes().

So wbc.nr_to_write <=0 indicates we are interrupted by the quota and
should revisit generic_sync_sb_inodes() to check for more io (which will
_normally_ find more dirty pages to write).

> Or just have the section below brought up above this
> break check and do:
> 
> if (!list_empty(&sb->s_more_io) || !list_empty(&sb->s_io))
>         wbc->more_io = 1;
> 
> Am I just misunderstanding the intent of more_io ?

It should be OK. I agree on the change if it makes the logic more
straightforward.

Thanks,
Fengguang
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