Re: [PATCH 1/6] mm, compaction: more robust check for scanners meeting

[Michal Nazarewicz]

On Wed, Jun 10 2015, Vlastimil Babka wrote:
> Compaction should finish when the migration and free scanner meet, i.e. they
> reach the same pageblock. Currently however, the test in compact_finished()
> simply just compares the exact pfns, which may yield a false negative when the
> free scanner position is in the middle of a pageblock and the migration scanner
> reaches the beginning of the same pageblock.
>
> This hasn't been a problem until commit e14c720efdd7 ("mm, compaction: remember
> position within pageblock in free pages scanner") allowed the free scanner
> position to be in the middle of a pageblock between invocations.  The hot-fix
> 1d5bfe1ffb5b ("mm, compaction: prevent infinite loop in compact_zone")
> prevented the issue by adding a special check in the migration scanner to
> satisfy the current detection of scanners meeting.
>
> However, the proper fix is to make the detection more robust. This patch
> introduces the compact_scanners_met() function that returns true when the free
> scanner position is in the same or lower pageblock than the migration scanner.
> The special case in isolate_migratepages() introduced by 1d5bfe1ffb5b is
> removed.
>
> Suggested-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>

Acked-by: 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>
> ---
>  mm/compaction.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 16e1b57..d46aaeb 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -902,6 +902,16 @@ static bool suitable_migration_target(struct page *page)
>  }
>  
>  /*
> + * Test whether the free scanner has reached the same or lower pageblock than
> + * the migration scanner, and compaction should thus terminate.
> + */
> +static inline bool compact_scanners_met(struct compact_control *cc)
> +{
> +	return (cc->free_pfn >> pageblock_order)
> +		<= (cc->migrate_pfn >> pageblock_order);
> +}
> +
> +/*
>   * Based on information in the current compact_control, find blocks
>   * suitable for isolating free pages from and then isolate them.
>   */
> @@ -1131,12 +1141,8 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>  	}
>  
>  	acct_isolated(zone, cc);
> -	/*
> -	 * Record where migration scanner will be restarted. If we end up in
> -	 * the same pageblock as the free scanner, make the scanners fully
> -	 * meet so that compact_finished() terminates compaction.
> -	 */
> -	cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
> +	/* Record where migration scanner will be restarted. */
> +	cc->migrate_pfn = low_pfn;
>  
>  	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
>  }
> @@ -1151,7 +1157,7 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
>  		return COMPACT_PARTIAL;
>  
>  	/* Compaction run completes if the migrate and free scanner meet */
> -	if (cc->free_pfn <= cc->migrate_pfn) {
> +	if (compact_scanners_met(cc)) {
>  		/* Let the next compaction start anew. */
>  		zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
>  		zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
> @@ -1380,7 +1386,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>  			 * migrate_pages() may return -ENOMEM when scanners meet
>  			 * and we want compact_finished() to detect it
>  			 */
> -			if (err == -ENOMEM && cc->free_pfn > cc->migrate_pfn) {
> +			if (err == -ENOMEM && !compact_scanners_met(cc)) {
>  				ret = COMPACT_PARTIAL;
>  				goto out;
>  			}
> -- 
> 2.1.4
>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  MichaÅ âmina86â Nazarewicz    (o o)
ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>--ooO--(_)--Ooo--
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