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@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 :)
/* 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@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
--
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>