On 06/09/2014 05:26 PM, Vlastimil Babka wrote:
> isolate_freepages_block() rechecks if the pageblock is suitable to be a target
> for migration after it has taken the zone->lock. However, the check has been
> optimized to occur only once per pageblock, and compact_checklock_irqsave()
> might be dropping and reacquiring lock, which means somebody else might have
> changed the pageblock's migratetype meanwhile.
>
> Furthermore, nothing prevents the migratetype to change right after
> isolate_freepages_block() has finished isolating. Given how imperfect this is,
> it's simpler to just rely on the check done in isolate_freepages() without
> lock, and not pretend that the recheck under lock guarantees anything. It is
> just a heuristic after all.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> I suggest folding mm-compactionc-isolate_freepages_block-small-tuneup.patch into this
>
> mm/compaction.c | 13 -------------
> 1 file changed, 13 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5175019..b73b182 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -276,7 +276,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> struct page *cursor, *valid_page = NULL;
> unsigned long flags;
> bool locked = false;
> - bool checked_pageblock = false;
>
> cursor = pfn_to_page(blockpfn);
>
> @@ -307,18 +306,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> if (!locked)
> break;
>
> - /* Recheck this is a suitable migration target under lock */
> - if (!strict && !checked_pageblock) {
> - /*
> - * We need to check suitability of pageblock only once
> - * and this isolate_freepages_block() is called with
> - * pageblock range, so just check once is sufficient.
> - */
> - checked_pageblock = true;
> - if (!suitable_migration_target(page))
> - break;
> - }
> -
> /* Recheck this is a buddy page under lock */
> if (!PageBuddy(page))
> goto isolate_fail;
>
--
Thanks.
Zhang Yanfei
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