[PATCH 3.16 184/294] fix unbalanced page refcounting in bio_map_user_iov

[Ben Hutchings]

3.16.50-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Vitaly Mayatskikh <v.mayatskih@xxxxxxxxx>

commit 95d78c28b5a85bacbc29b8dba7c04babb9b0d467 upstream.

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@xxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 block/bio.c | 8 ++++++++
 1 file changed, 8 insertions(+)

--- a/block/bio.c
+++ b/block/bio.c
@@ -1337,6 +1337,7 @@ static struct bio *__bio_map_user_iov(st
 		offset = uaddr & ~PAGE_MASK;
 		for (j = cur_page; j < page_limit; j++) {
 			unsigned int bytes = PAGE_SIZE - offset;
+			unsigned short prev_bi_vcnt = bio->bi_vcnt;
 
 			if (len <= 0)
 				break;
@@ -1351,6 +1352,13 @@ static struct bio *__bio_map_user_iov(st
 					    bytes)
 				break;
 
+			/*
+			 * check if vector was merged with previous
+			 * drop page reference if needed
+			 */
+			if (bio->bi_vcnt == prev_bi_vcnt)
+				put_page(pages[j]);
+
 			len -= bytes;
 			offset = 0;
 		}