[PATCH] fix page_count in ->iomap_migrate_page()

[zhangjun <openzhangj@gmail.com>]

IOMAP uses PG_private a little different with buffer_head based
filesystem.
It uses it as marker and when set, the page counter is not incremented,
migrate_page_move_mapping() assumes that PG_private indicates a counter
of +1.
so, we have to pass a extra count of -1 to migrate_page_move_mapping()
if the flag is set.

Signed-off-by: zhangjun <openzhangj@gmail.com>
---
 fs/iomap.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/iomap.c b/fs/iomap.c
index 64ce240..352e58a 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -544,8 +544,17 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
 		struct page *page, enum migrate_mode mode)
 {
 	int ret;
+	int extra_count = 0;
 
-	ret = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
+	/*
+	 * IOMAP uses PG_private as marker and does not raise the page counter.
+	 * migrate_page_move_mapping() expects a incremented counter if PG_private
+	 * is set. Therefore pass -1 as extra_count for this case.
+	 */
+	if (page_has_private(page))
+		extra_count = -1;
+	ret = migrate_page_move_mapping(mapping, newpage, page,
+		       NULL, mode, extra_count);
 	if (ret != MIGRATEPAGE_SUCCESS)
 		return ret;
 
-- 
2.7.4