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Date: Mon, 24 Nov 2008 17:05:03 -0800 (PST)
From: David Rientjes <rientjes@google.com>
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To: Randy Dunlap <randy.dunlap@oracle.com>
cc: Andrew Morton <akpm@linux-foundation.org>, peterz@infradead.org,
       david@fromorbit.com, cl@linux-foundation.org,
       linux-kernel@vger.kernel.org
Subject: Re: [patch 2/2] mm: add dirty_background_bytes and dirty_bytes
 sysctls
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On Mon, 24 Nov 2008, Randy Dunlap wrote:

> >> -	if (background_ratio >= dirty_ratio)
> >> -		background_ratio = dirty_ratio / 2;
> >> +		dirty_ratio = vm_dirty_ratio;
> >> +		if (dirty_ratio < 5)
> >> +			dirty_ratio = 5;
> >> +		dirty = (dirty_ratio * available_memory) / 100;
> >> +	}
> >> +
> >> +	if (dirty_background_bytes)
> >> +		background = (dirty_background_bytes + PAGE_SIZE) / PAGE_SIZE;
> > 
> > DIV_ROUND_UP()?
> 
> Note that DIV_ROUND_UP() would make that
> 
> 		background = (dirty_background_byte + PAGE_SIZE - 1) / PAGE_SIZE;
> 
> but that's usually what is needed/intended.
> 

Good point.  If we were to do this, then dirty_bytes and 
dirty_background_bytes should use 1 instead of 0 as the minimum acceptable 
value in proc_dointvec_minmax().  Otherwise, get_dirty_limits() would 
return 0 pages for the dirty limit when 0 is written to either file since 
integer division truncates and we'd be in a constant state of writeback :)