linux-kernel - Re: [PATCH] mm: prevent divide error for small values of vm_dirty

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Date:	Wed, 29 Apr 2009 13:02:27 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrea Righi <righi.andrea@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	Dave Chinner <david@...morbit.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: prevent divide error for small values of
 vm_dirty_bytes

On Wed, 29 Apr 2009, Andrea Righi wrote:

> On Wed, Apr 29, 2009 at 10:44:36AM +0200, Peter Zijlstra wrote:
> > On Wed, 2009-04-29 at 10:29 +0200, Andrea Righi wrote:
> > > Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> > > potential division by 0 (like the following) in get_dirty_limits().
> > 
> > isn't changing the .extra1 in the sysctl table a better fix?
> 
> I thought about that and probably you're right. But, in the first case
> if the user writes a value < 2*PAGE_SIZE vm_dirty_bytes is explicitly
> set to 2*PAGE_SIZE. In the other case the value is just ignored.
> 
> Maybe the second case could generate some unexpected behaviours for
> the user. For example, the user writes X < 2*PAGE_SIZE to
> /proc/sys/vm/dirty_bytes, and could think that the system has started to
> use this new configuration, but it isn't actually.
> 
> So, probably the best way is to change .extra1 and add a note in the
> documentation (see below).
> 

This is only caused by bdi, not by the dirty ratio itself; shouldn't the 
appropriate fix be in calc_period_shift()?  In other words, allow 
dirty thresholds to be less than 2 * PAGE_SIZE but prevent the bdi 
fraction from having a zero denominator?
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