[PATCH AUTOSEL 5.1 77/95] block: fix page leak when merging to same page

[Sasha Levin]

From: Christoph Hellwig <hch@xxxxxx>

[ Upstream commit 4569180495600ac59f5cd27f67242a6cb51254f3 ]

When multiple iovecs reference the same page, each get_user_page call
will add a reference to the page.  But once we've created the bio that
information gets lost and only a single reference will be dropped after
I/O completion.  Use the same_page information returned from
__bio_try_merge_page to drop additional references to pages that were
already present in the bio.

Based on a patch from Ming Lei.

Link: https://lkml.org/lkml/2019/4/23/64
Fixes: 576ed913 ("block: use bio_add_page in bio_iov_iter_get_pages")
Reported-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 block/bio.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index a3c80a6c1fe5..80a25c245109 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -884,6 +884,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
 	struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
 	struct page **pages = (struct page **)bv;
+	bool same_page = false;
 	ssize_t size, left;
 	unsigned len, i;
 	size_t offset;
@@ -904,8 +905,15 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		struct page *page = pages[i];
 
 		len = min_t(size_t, PAGE_SIZE - offset, left);
-		if (WARN_ON_ONCE(bio_add_page(bio, page, len, offset) != len))
-			return -EINVAL;
+
+		if (__bio_try_merge_page(bio, page, len, offset, &same_page)) {
+			if (same_page)
+				put_page(page);
+		} else {
+			if (WARN_ON_ONCE(bio_full(bio)))
+                                return -EINVAL;
+			__bio_add_page(bio, page, len, offset);
+		}
 		offset = 0;
 	}
 
-- 
2.20.1