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SubjectRe: [patch] mm: compaction: prevent division-by-zero during user-requested compaction
On Tue, Jan 18, 2011 at 06:06:52PM +0100, Johannes Weiner wrote:
> Up until '3e7d344 mm: vmscan: reclaim order-0 and use compaction
> instead of lumpy reclaim', compaction skipped calculating the
> fragmentation index of a zone when compaction was explicitely
> requested through the procfs knob.
>
> However, when compaction_suitable was introduced, it did not come with
> an extra check for order == -1, set on explicit compaction requests,
> and passed this order on to the fragmentation index calculation, where
> it overshifts the number of requested pages, leading to a division by
> zero.
>
> This patch makes sure that order == -1 is recognized as the flag it is
> rather than passing it along as valid order parameter.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

It could do with a comment saying that order == -1 is expected when
compacting via /proc/sys/vm/compact_memory but otherwise;

Reviewed-by: Mel Gorman <mel@csn.ul.ie>

> ---
> mm/compaction.c | 3 +++
> 1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 6d592a0..114c145 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -453,6 +453,9 @@ unsigned long compaction_suitable(struct zone *zone, int order)
> if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
> return COMPACT_SKIPPED;
>
> + if (order == -1)
> + return COMPACT_CONTINUE;
> +
> /*
> * fragmentation index determines if allocation failures are due to
> * low memory or external fragmentation
> --
> 1.7.3.4
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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