mm: compaction: handle incorrect MIGRATE_UNMOVABLE type pageblocks

When MIGRATE_UNMOVABLE pages are freed from MIGRATE_UNMOVABLE type
pageblock (and some MIGRATE_MOVABLE pages are left in it) waiting until an
allocation takes ownership of the block may take too long.  The type of
the pageblock remains unchanged so the pageblock cannot be used as a
migration target during compaction.

Fix it by:

* Adding enum compact_mode (COMPACT_ASYNC_[MOVABLE,UNMOVABLE], and
  COMPACT_SYNC) and then converting sync field in struct compact_control
  to use it.

* Adding nr_pageblocks_skipped field to struct compact_control and
  tracking how many destination pageblocks were of MIGRATE_UNMOVABLE type.
   If COMPACT_ASYNC_MOVABLE mode compaction ran fully in
  try_to_compact_pages() (COMPACT_COMPLETE) it implies that there is not a
  suitable page for allocation.  In this case then check how if there were
  enough MIGRATE_UNMOVABLE pageblocks to try a second pass in
  COMPACT_ASYNC_UNMOVABLE mode.

* Scanning the MIGRATE_UNMOVABLE pageblocks (during COMPACT_SYNC and
  COMPACT_ASYNC_UNMOVABLE compaction modes) and building a count based on
  finding PageBuddy pages, page_count(page) == 0 or PageLRU pages.  If all
  pages within the MIGRATE_UNMOVABLE pageblock are in one of those three
  sets change the whole pageblock type to MIGRATE_MOVABLE.

My particular test case (on a ARM EXYNOS4 device with 512 MiB, which means
131072 standard 4KiB pages in 'Normal' zone) is to:

- allocate 120000 pages for kernel's usage
- free every second page (60000 pages) of memory just allocated
- allocate and use 60000 pages from user space
- free remaining 60000 pages of kernel memory
  (now we have fragmented memory occupied mostly by user space pages)
- try to allocate 100 order-9 (2048 KiB) pages for kernel's usage

The results:
- with compaction disabled I get 11 successful allocations
- with compaction enabled - 14 successful allocations
- with this patch I'm able to get all 100 successful allocations

NOTE: If we can make kswapd aware of order-0 request during compaction, we
can enhance kswapd with changing mode to COMPACT_ASYNC_FULL
(COMPACT_ASYNC_MOVABLE + COMPACT_ASYNC_UNMOVABLE).  Please see the
following thread:

	http://marc.info/?l=linux-mm&m=133552069417068&w=2

[minchan@kernel.org: minor cleanups]
Cc: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index 51a90b7..e988037 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_COMPACTION_H
 #define _LINUX_COMPACTION_H
 
+#include <linux/node.h>
+
 /* Return values for compact_zone() and try_to_compact_pages() */
 /* compaction didn't start as it was not possible or direct reclaim was more suitable */
 #define COMPACT_SKIPPED		0
@@ -11,6 +13,23 @@
 /* The full zone was compacted */
 #define COMPACT_COMPLETE	3
 
+/*
+ * compaction supports three modes
+ *
+ * COMPACT_ASYNC_MOVABLE uses asynchronous migration and only scans
+ *    MIGRATE_MOVABLE pageblocks as migration sources and targets.
+ * COMPACT_ASYNC_UNMOVABLE uses asynchronous migration and only scans
+ *    MIGRATE_MOVABLE pageblocks as migration sources.
+ *    MIGRATE_UNMOVABLE pageblocks are scanned as potential migration
+ *    targets and convers them to MIGRATE_MOVABLE if possible
+ * COMPACT_SYNC uses synchronous migration and scans all pageblocks
+ */
+enum compact_mode {
+	COMPACT_ASYNC_MOVABLE,
+	COMPACT_ASYNC_UNMOVABLE,
+	COMPACT_SYNC,
+};
+
 #ifdef CONFIG_COMPACTION
 extern int sysctl_compact_memory;
 extern int sysctl_compaction_handler(struct ctl_table *table, int write,