Re: [PATCH 01/10] mm, compaction: do not recheck suitable_migration_target under lock

[Zhang Yanfei]

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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