linux-kernel - [PATCH] fix unbalanced page refcounting in bio_map_user

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Date:   Fri, 22 Sep 2017 01:18:39 -0400
From:   Vitaly Mayatskikh <v.mayatskih@...il.com>
To:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>
Subject: [PATCH] fix unbalanced page refcounting in bio_map_user_iov

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@...il.com>

diff --git a/block/bio.c b/block/bio.c
index b38e962fa83e..10cd3b6bed27 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1383,6 +1383,7 @@ struct bio *bio_map_user_iov(struct request_queue *q,
 		offset = offset_in_page(uaddr);
 		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;
@@ -1397,6 +1398,13 @@ struct bio *bio_map_user_iov(struct request_queue *q,
 					    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;
 		}

-- 
wbr, Vitaly