linux-kernel - Re: [PATCH 3/7] writeback: fix dirtied pages accounting on sub-page writes

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Date:	Thu, 8 Dec 2011 10:44:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] writeback: fix dirtied pages accounting on
 sub-page writes

On Thu, Dec 08, 2011 at 12:07:53AM +0800, Jan Kara wrote:
> On Wed 07-12-11 20:08:18, Wu Fengguang wrote:
> > On Wed, Dec 07, 2011 at 06:53:40PM +0800, Jan Kara wrote:
> > > On Mon 28-11-11 21:53:41, Wu Fengguang wrote:
> > > > When dd in 512bytes, generic_perform_write() calls
> > > > balance_dirty_pages_ratelimited() 8 times for the same page, but
> > > > obviously the page is only dirtied once.
> > >   Actually, for ppc where pages can be 64 KB the problem is even worse.
> > 
> > Ah yes.
> > 
> > > > Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.
> > >   I was wondering about one more thing - couldn't we rather check in
> > > generic_perform_write() whether the page was dirty before calling
> > > ->write_end and call balance_dirty_pages_ratelimited() only if it wasn't?
> > 
> > Cough.. the very original version does that exactly, then you raised
> > some concern here:
> > 
> >         https://lkml.org/lkml/2011/4/13/554
> > 
> > The discussion goes on and eventually I get to the current version
> > that looks most acceptable in the three options.
>   Good point. I should have researched web (or my memory) more closely ;)
> Thanks for the pointer - it has reminded me why using PageDirty isn't quite
> perfect.
> 
> > >   For generic_perform_write() it doesn't really matter that much since we
> > > do things page-by-page anyway but other callers could be more efficient...
> > 
> > That's right.
>   You can add:
> Acked-by: Jan Kara <jack@...e.cz>

OK, thanks!

Fengguang
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