[PATCH v3 14/17] mm, compaction: create compact_gap wrapper

[Vlastimil Babka <vbabka@suse.cz>]

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.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 include/linux/compaction.h | 16 ++++++++++++++++
 mm/compaction.c            |  7 +++----
 mm/vmscan.c                |  4 ++--
 3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index a6b3d5d2ae53..67a3372c4753 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -58,6 +58,22 @@ enum compact_result {
 
 struct alloc_context; /* in mm/internal.h */
 
+/*
+ * Number of free order-0 pages that should be available above given watermark
+ * to make sure compaction has reasonable chance of not running out of free
+ * pages that it needs to isolate as migration target during its work.
+ */
+static inline unsigned long compact_gap(unsigned int order)
+{
+	/*
+	 * Although all the isolations for migration are temporary, compaction
+	 * may have up to 1 << order pages on its list and then try to split
+	 * an (order - 1) free page. At that point, a gap of 1 << order might
+	 * not be enough, so it's safer to require twice that amount.
+	 */
+	return 2UL << order;
+}
+
 #ifdef CONFIG_COMPACTION
 extern int sysctl_compact_memory;
 extern int sysctl_compaction_handler(struct ctl_table *table, int write,
diff --git a/mm/compaction.c b/mm/compaction.c
index 371760a85085..c1ce7c2abe05 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1395,11 +1395,10 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 		return COMPACT_PARTIAL;
 
 	/*
-	 * 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 21d417ccff69..484ff05d5a8f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2351,7 +2351,7 @@ static inline bool should_continue_reclaim(struct zone *zone,
 	 * 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 = zone_page_state(zone, NR_INACTIVE_FILE);
 	if (get_nr_swap_pages() > 0)
 		inactive_lru_pages += zone_page_state(zone, NR_INACTIVE_ANON);
@@ -2478,7 +2478,7 @@ static inline bool compaction_ready(struct zone *zone, int order, int classzone_
 	 */
 	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
 			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
-	watermark = high_wmark_pages(zone) + balance_gap + (2UL << order);
+	watermark = high_wmark_pages(zone) + balance_gap + compact_gap(order);
 	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, classzone_idx);
 
 	/*
-- 
2.8.4

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