[PATCH] compaction: add def_blk_aops migrate function for memory compaction

[zhouxianrong]

From: zhouxianrong <zhouxianrong@xxxxxxxxxx>

the reason for why to do this is based on below factors.

1. larg file read/write operations with order 0 can fragmentize
   memory rapidly.

2. when a special filesystem does not supply migratepage callback,
   kernel would fallback to default function fallback_migrate_page.
   but fallback_migrate_page could not migrate diry page nicely;
   specially kcompactd with MIGRATE_SYNC_LIGHT could not migrate
   diry pages due to this until clear_page_dirty_for_io in some
   procedure. i think it is not suitable here in this scenario.
   for dirty pages we should migrate it rather than skip or writeout
   it in kcomapctd with MIGRATE_SYNC_LIGHT. i think this problem is
   for all filesystem without migratepage not only for block device fs. 
   
so for compaction under large file writing supply migratepage for
def_blk_aops.

Signed-off-by: zhouxianrong <zhouxianrong@xxxxxxxxxx>
---
 fs/block_dev.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 1c62845..9343b60 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -2062,6 +2062,9 @@ static int blkdev_writepages(struct address_space *mapping,
 	.releasepage	= blkdev_releasepage,
 	.direct_IO	= blkdev_direct_IO,
 	.is_dirty_writeback = buffer_check_dirty_writeback,
+#ifdef CONFIG_MIGRATION
+	.migratepage = buffer_migrate_page,
+#endif
 };
 
 #define	BLKDEV_FALLOC_FL_SUPPORTED					\
-- 
1.7.9.5