linux-kernel - Re: [patch 1/2] mm: page-writeback: fix dirty_balance_reserve subtraction from dirtyable memory

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Date:	Fri, 24 Jan 2014 18:05:09 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Tejun Heo <tj@...nel.org>, Mel Gorman <mgorman@...e.de>,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm: page-writeback: fix dirty_balance_reserve subtraction
 from dirtyable memory

On 01/24/2014 05:03 PM, Johannes Weiner wrote:
> The dirty_balance_reserve is an approximation of the fraction of free
> pages that the page allocator does not make available for page cache
> allocations.  As a result, it has to be taken into account when
> calculating the amount of "dirtyable memory", the baseline to which
> dirty_background_ratio and dirty_ratio are applied.
> 
> However, currently the reserve is subtracted from the sum of free and
> reclaimable pages, which is non-sensical and leads to erroneous
> results when the system is dominated by unreclaimable pages and the
> dirty_balance_reserve is bigger than free+reclaimable.  In that case,
> at least the already allocated cache should be considered dirtyable.
> 
> Fix the calculation by subtracting the reserve from the amount of free
> pages, then adding the reclaimable pages on top.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Rik van Riel <riel@...hat.com>

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