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, Michał “mina86” Nazarewicz (o o) ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>--ooO--(_)--Ooo-- -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href