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

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Date:	Wed, 7 Dec 2011 20:08:18 +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 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.

>   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.

Thanks,
Fengguang

> > Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  mm/page-writeback.c |   13 +++++--------
> >  1 file changed, 5 insertions(+), 8 deletions(-)
> > 
> > --- linux-next.orig/mm/page-writeback.c	2011-11-28 21:23:20.000000000 +0800
> > +++ linux-next/mm/page-writeback.c	2011-11-28 21:23:23.000000000 +0800
> > @@ -1239,8 +1239,6 @@ void balance_dirty_pages_ratelimited_nr(
> >  	if (bdi->dirty_exceeded)
> >  		ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
> >  
> > -	current->nr_dirtied += nr_pages_dirtied;
> > -
> >  	preempt_disable();
> >  	/*
> >  	 * This prevents one CPU to accumulate too many dirtied pages without
> > @@ -1251,12 +1249,9 @@ void balance_dirty_pages_ratelimited_nr(
> >  	p =  &__get_cpu_var(bdp_ratelimits);
> >  	if (unlikely(current->nr_dirtied >= ratelimit))
> >  		*p = 0;
> > -	else {
> > -		*p += nr_pages_dirtied;
> > -		if (unlikely(*p >= ratelimit_pages)) {
> > -			*p = 0;
> > -			ratelimit = 0;
> > -		}
> > +	else if (unlikely(*p >= ratelimit_pages)) {
> > +		*p = 0;
> > +		ratelimit = 0;
> >  	}
> >  	/*
> >  	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
> > @@ -1749,6 +1744,8 @@ void account_page_dirtied(struct page *p
> >  		__inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> >  		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> >  		task_io_account_write(PAGE_CACHE_SIZE);
> > +		current->nr_dirtied++;
> > +		this_cpu_inc(bdp_ratelimits);
> >  	}
> >  }
> >  EXPORT_SYMBOL(account_page_dirtied);
> > 
> > 
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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