Re: [PATCH 1/2] mm/compaction: make isolate_freepages start at pageblock boundary

[Joonsoo Kim]

On Tue, Apr 15, 2014 at 11:18:26AM +0200, Vlastimil Babka wrote:
> The compaction freepage scanner implementation in isolate_freepages() starts
> by taking the current cc->free_pfn value as the first pfn. In a for loop, it
> scans from this first pfn to the end of the pageblock, and then subtracts
> pageblock_nr_pages from the first pfn to obtain the first pfn for the next
> for loop iteration.
> 
> This means that when cc->free_pfn starts at offset X rather than being aligned
> on pageblock boundary, the scanner will start at offset X in all scanned
> pageblock, ignoring potentially many free pages. Currently this can happen when
> a) zone's end pfn is not pageblock aligned, or
> b) through zone->compact_cached_free_pfn with CONFIG_HOLES_IN_ZONE enabled and
>    a hole spanning the beginning of a pageblock
> 
> This patch fixes the problem by aligning the initial pfn in isolate_freepages()
> to pageblock boundary. This also allows to replace the end-of-pageblock
> alignment within the for loop with a simple pageblock_nr_pages increment.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reported-by: Heesub Shin <heesub.shin@xxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  mm/compaction.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)

Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
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