On Thu, 6 Feb 2014, Vlastimil Babka 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@xxxxxxxxxx> > > --- > > 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 :) > That's why I sent the fix for page_count(). The "racy check" the comment eludes to above concerns the fact that page_count() and page_mapcount() can change out from under us before isolation and if we had not avoided isolating them that they would have been migratable later. We accept that as a consequence of doing this in a lockless way without page references. -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>