block: fix page leak when merging to same page

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@gibson.dropbear.id.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bio.c b/block/bio.c
index daa1c1a..ce797d7 100644 (file)
--- a/block/bio.c
+++ b/block/bio.c
@@ -896,6 +896,7 @@ 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;
@@ -916,8 +917,15 @@ 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;
        }