mm/compaction: make isolate_freepages start at pageblock boundary

The compaction freepage scanner implementation in isolate_freepages()
starts by taking the current cc->free_pfn value as the first pfn.  In a
for loop, it scans from this first pfn to the end of the pageblock, and
then subtracts pageblock_nr_pages from the first pfn to obtain the first
pfn for the next for loop iteration.

This means that when cc->free_pfn starts at offset X rather than being
aligned on pageblock boundary, the scanner will start at offset X in all
scanned pageblock, ignoring potentially many free pages.  Currently this
can happen when

 a) zone's end pfn is not pageblock aligned, or

 b) through zone->compact_cached_free_pfn with CONFIG_HOLES_IN_ZONE
    enabled and a hole spanning the beginning of a pageblock

This patch fixes the problem by aligning the initial pfn in
isolate_freepages() to pageblock boundary.  This also permits replacing
the end-of-pageblock alignment within the for loop with a simple
pageblock_nr_pages increment.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Heesub Shin <heesub.shin@samsung.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Dongjun Shin <d.j.shin@samsung.com>
Cc: Sunghwan Yun <sunghwan.yun@samsung.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 10 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 37f976287068..627dc2e4320f 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -671,16 +671,20 @@ static void isolate_freepages(struct zone *zone,
                                 struct compact_control *cc)
 {
         struct page *page;
-        unsigned long high_pfn, low_pfn, pfn, z_end_pfn, end_pfn;
+        unsigned long high_pfn, low_pfn, pfn, z_end_pfn;
         int nr_freepages = cc->nr_freepages;
         struct list_head *freelist = &cc->freepages;
 
         /*
          * Initialise the free scanner. The starting point is where we last
-         * scanned from (or the end of the zone if starting). The low point
-         * is the end of the pageblock the migration scanner is using.
+         * successfully isolated from, zone-cached value, or the end of the
+         * zone when isolating for the first time. We need this aligned to
+         * the pageblock boundary, because we do pfn -= pageblock_nr_pages
+         * in the for loop.
+         * The low boundary is the end of the pageblock the migration scanner
+         * is using.
          */
-        pfn = cc->free_pfn;
+        pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
         low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
 
         /*
@@ -700,6 +704,7 @@ static void isolate_freepages(struct zone *zone,
         for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
                                         pfn -= pageblock_nr_pages) {
                 unsigned long isolated;
+                unsigned long end_pfn;
 
                 /*
                  * This can iterate a massively long zone without finding any
@@ -734,13 +739,10 @@ static void isolate_freepages(struct zone *zone,
                 isolated = 0;
 
                 /*
-                 * As pfn may not start aligned, pfn+pageblock_nr_page
-                 * may cross a MAX_ORDER_NR_PAGES boundary and miss
-                 * a pfn_valid check. Ensure isolate_freepages_block()
-                 * only scans within a pageblock
+                 * Take care when isolating in last pageblock of a zone which
+                 * ends in the middle of a pageblock.
                  */
-                end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
-                end_pfn = min(end_pfn, z_end_pfn);
+                end_pfn = min(pfn + pageblock_nr_pages, z_end_pfn);
                 isolated = isolate_freepages_block(cc, pfn, end_pfn,
                                                    freelist, false);
                 nr_freepages += isolated;