mm: have order > 0 compaction start off where it left

This patch makes the comment for cc->wrapped longer, explaining what is
really going on.  It also incorporates the comment fix pointed out by
Minchan.

Additionally, Minchan found that, when no pages get isolated, high_pfn
could be a value that is much lower than desired, which might potentially
cause compaction to skip a range of pages.

Only assign zone->compact_cache_free_pfn if we actually isolated free
pages for compaction.

Split out the calculation to get the start of the last page block in a
zone into its own, commented function.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Jim Schutt <jaschut@sandia.gov>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ffcc32f8ae0d29471a442e9e53d5436d81ba2d1f..1aeadce4d56eb5d747f740dddf4d27368c37a8ab 100644 (file)
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -369,7 +369,7 @@ struct zone {
        spinlock_t              lock;
        int                     all_unreclaimable; /* All pages pinned */
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
-       /* pfn where the last order > 0 compaction isolated free pages */
+       /* pfn where the last incremental compaction isolated free pages */
        unsigned long           compact_cached_free_pfn;
 #endif
 #ifdef CONFIG_MEMORY_HOTPLUG

diff --git a/mm/compaction.c b/mm/compaction.c
index fe990075eeb9dcb7954cf422bb02142244c264ea..ed851e4d7f8482379a77202f9c5da3ed7fb489fa 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -472,10 +472,11 @@ static void isolate_freepages(struct zone *zone,
                 * looking for free pages, the search will restart here as
                 * page migration may have returned some pages to the allocator
                 */
-               if (isolated)
+               if (isolated) {
                        high_pfn = max(high_pfn, pfn);
-               if (cc->order > 0)
-                       zone->compact_cached_free_pfn = high_pfn;
+                       if (cc->order > 0)
+                               zone->compact_cached_free_pfn = high_pfn;
+               }
        }
 
        /* split_free_page does not map the pages */
@@ -569,6 +570,21 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
        return ISOLATE_SUCCESS;
 }
 
+/*
+ * Returns the start pfn of the laste page block in a zone.
+ * This is the starting point for full compaction of a zone.
+ * Compaction searches for free pages from the end of each zone,
+ * while isolate_freepages_block scans forward inside each page
+ * block.
+ */
+static unsigned long start_free_pfn(struct zone *zone)
+{
+       unsigned long free_pfn;
+       free_pfn = zone->zone_start_pfn + zone->spanned_pages;
+       free_pfn &= ~(pageblock_nr_pages-1);
+       return free_pfn;
+}
+
 static int compact_finished(struct zone *zone,
                            struct compact_control *cc)
 {
@@ -587,10 +603,9 @@ static int compact_finished(struct zone *zone,
        if (cc->free_pfn <= cc->migrate_pfn) {
                if (cc->order > 0 && !cc->wrapped) {
                        /* We started partway through; restart at the end. */
-                       unsigned long free_pfn;
-                       free_pfn = zone->zone_start_pfn + zone->spanned_pages;
-                       free_pfn &= ~(pageblock_nr_pages-1);
+                       unsigned long free_pfn = start_free_pfn(zone);
                        zone->compact_cached_free_pfn = free_pfn;
+                       cc->free_pfn = free_pfn;
                        cc->wrapped = 1;
                        return COMPACT_CONTINUE;
                }
@@ -703,8 +718,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
                cc->start_free_pfn = cc->free_pfn;
        } else {
                /* Order == -1 starts at the end of the zone. */
-               cc->free_pfn = cc->migrate_pfn + zone->spanned_pages;
-               cc->free_pfn &= ~(pageblock_nr_pages-1);
+               cc->free_pfn = start_free_pfn(zone);
        }
 
        migrate_prep_local();

diff --git a/mm/internal.h b/mm/internal.h
index 0b724610d207f653e9f6d3740603dce45dd6972f..da6b9b2ed3fc429443add0dc2349098b83ee9ea8 100644 (file)
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -121,7 +121,11 @@ struct compact_control {
        unsigned long start_free_pfn;   /* where we started the search */
        unsigned long migrate_pfn;      /* isolate_migratepages search base */
        bool sync;                      /* Synchronous migration */
-       bool wrapped;                   /* Last round for order>0 compaction */
+       bool wrapped;                   /* Order > 0 compactions are
+                                          incremental, once free_pfn
+                                          and migrate_pfn meet, we restart
+                                          from the top of the zone;
+                                          remember we wrapped around. */
 
        int order;                      /* order a direct compactor needs */
        int migratetype;                /* MOVABLE, RECLAIMABLE etc */