mm, compaction: create compact_gap wrapper
Compaction uses a watermark gap of (2UL << order) pages at various
places and it's not immediately obvious why. Abstract it through a
compact_gap() wrapper to create a single place with a thorough
explanation.
[vbabka@suse.cz: clarify the comment of compact_gap()]
Link: http://lkml.kernel.org/r/7b6aed1f-fdf8-2063-9ff4-bbe4de712d37@suse.cz
Link: http://lkml.kernel.org/r/20160810091226.6709-9-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/compaction.c b/mm/compaction.c
index e2618ac..bbf41ee 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1391,11 +1391,10 @@
return COMPACT_SUCCESS;
/*
- * Watermarks for order-0 must be met for compaction. Note the 2UL.
- * This is because during migration, copies of pages need to be
- * allocated and for a short time, the footprint is higher
+ * Watermarks for order-0 must be met for compaction to be able to
+ * isolate free pages for migration targets.
*/
- watermark = low_wmark_pages(zone) + (2UL << order);
+ watermark = low_wmark_pages(zone) + compact_gap(order);
if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
alloc_flags, wmark_target))
return COMPACT_SKIPPED;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 981fc84..2a6978a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2480,7 +2480,7 @@
* If we have not reclaimed enough pages for compaction and the
* inactive lists are large enough, continue reclaiming
*/
- pages_for_compaction = (2UL << sc->order);
+ pages_for_compaction = compact_gap(sc->order);
inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
if (get_nr_swap_pages() > 0)
inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
@@ -2612,7 +2612,7 @@
* there is a buffer of free pages available to give compaction
* a reasonable chance of completing and allocating the page
*/
- watermark = high_wmark_pages(zone) + (2UL << sc->order);
+ watermark = high_wmark_pages(zone) + compact_gap(sc->order);
watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx);
/*
@@ -3169,7 +3169,7 @@
* excessive reclaim. Assume that a process requested a high-order
* can direct reclaim/compact.
*/
- if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
+ if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
sc->order = 0;
return sc->nr_scanned >= sc->nr_to_reclaim;