linux-kernel - Re: [patch 4/8] mm: allow not updating BDI stats in end_page

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Date:	Tue, 18 Mar 2008 14:59:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 4/8] mm: allow not updating BDI stats in
	end_page_writeback()

On Tue, 2008-03-18 at 14:58 +0100, Miklos Szeredi wrote:
> > > > So the thing that's in your way is that removing a page from the radix
> > > > tree doesn't imply its done writing. So perhaps we should make that
> > > > distinction instead?
> > > > 
> > > > So instead of conditionally do part of the accounting, never do it and
> > > > require something like: page_writeback_complete() to be called after a
> > > > successfull test_clear_page_writeback().
> > > 
> > > Yes, that's a possibility, but then normal filesystems miss out on the
> > > small optimization provided by doing the BDI accounting functions
> > > inside the same IRQ disabled region as the radix tree operation.
> > > Would that have any significant performance impact?
> > 
> > Yeah, realized that. Don't know, would have to measure it somehow...
> > some archs are rather slow with disabling IRQs, but we're talking about
> > writeout which should be dominated by the IO times.
> > 
> > Its just that your proposal exposes too much guts, I'd like the
> > interface to be a little higher level.
> 
> Well, but this is the kernel, you can't really make foolproof
> interfaces.  If we'll go with Andrew's suggestion, I'll add comments
> warning users about not touching those flags unless they know what
> they are doing, OK?

Yeah, I guess so :-)

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