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.

Link: http://lkml.kernel.org/r/20160810091226.6709-9-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
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: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index a1fba999472812d0240d5cef58bbed9e0e1d475e..e7f0d34a90fe1a0afbffc261f416b6f90a1a0caf 100644 (file)
--- 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 e2618ac062a660f97390f0fd43aa7b2a09754bfa..bbf41ee99142f88468d55a69f8ffbf2952514a46 100644 (file)
--- 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 f7f5eb4b9d6fc703709a2a2a5583bb0129563259..9d5f80b714f476b68b256fb85d931820e915ef5b 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2499,7 +2499,7 @@ 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);
@@ -2631,7 +2631,7 @@ 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);
 
        /*
@@ -3188,7 +3188,7 @@ 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;