Messages in this thread | | | Date | Thu, 06 Feb 2014 19:48:50 +0100 | From | Vlastimil Babka <> | Subject | Re: [patch v2] mm, compaction: avoid isolating pinned pages |
| |
On 5.2.2014 3:44, David Rientjes wrote: > Page migration will fail for memory that is pinned in memory with, for > example, get_user_pages(). In this case, it is unnecessary to take > zone->lru_lock or isolating the page and passing it to page migration > which will ultimately fail. > > This is a racy check, the page can still change from under us, but in > that case we'll just fail later when attempting to move the page. > > This avoids very expensive memory compaction when faulting transparent > hugepages after pinning a lot of memory with a Mellanox driver. > > On a 128GB machine and pinning ~120GB of memory, before this patch we > see the enormous disparity in the number of page migration failures > because of the pinning (from /proc/vmstat): > > compact_pages_moved 8450 > compact_pagemigrate_failed 15614415 > > 0.05% of pages isolated are successfully migrated and explicitly > triggering memory compaction takes 102 seconds. After the patch: > > compact_pages_moved 9197 > compact_pagemigrate_failed 7 > > 99.9% of pages isolated are now successfully migrated in this > configuration and memory compaction takes less than one second. > > Signed-off-by: David Rientjes <rientjes@google.com> > --- > v2: address page count issue per Joonsoo > > mm/compaction.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/mm/compaction.c b/mm/compaction.c > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -578,6 +578,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > continue; > } > > + /* > + * Migration will fail if an anonymous page is pinned in memory, > + * so avoid taking lru_lock and isolating it unnecessarily in an > + * admittedly racy check. > + */ > + if (!page_mapping(page) && > + page_count(page) > page_mapcount(page)) > + continue; > +
Hm this page_count() seems it could substantially increase the chance of race with prep_compound_page that your patch "mm, page_alloc: make first_page visible before PageTail" tries to fix :)
> /* Check if it is ok to still hold the lock */ > locked = compact_checklock_irqsave(&zone->lru_lock, &flags, > locked, cc); > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
| |