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>

3 files changed, 29 insertions, 7 deletions

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index a1fba9994728..585d55cb0dc0 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -58,6 +58,29 @@ 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
+         * free scanner 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. Note that the number of pages on the list is also
+         * effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
+         * that the migrate scanner can have isolated on migrate list, and free
+         * scanner is only invoked when the number of isolated free pages is
+         * lower than that. But it's not worth to complicate the formula here
+         * as a bigger gap for higher orders than strictly necessary can also
+         * improve chances of compaction success.
+         */
+        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 e2618ac062a6..bbf41ee99142 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1391,11 +1391,10 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
                 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 981fc84e7434..2a6978a07d56 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2480,7 +2480,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
          * 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 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
          * 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 @@ static bool kswapd_shrink_node(pg_data_t *pgdat,
          * 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;