[PATCH 1/4] block: bio: introduce helpers to get the 1st and last bvec

[Ming Lei <ming.lei@canonical.com>]

After bio splitting is introduced, the splitted bio can
be fast-cloned, which is correct because biovecs has become
immutable since v3.13.

Unfortunately bio_will_gap() isn't ready for this kind of change,
because it figures out the last bvec via 'bi_io_vec[prev->bi_vcnt - 1]'.

This patch introduces two helpers for getting the first and last
bvec of one bio.

Cc: stable@vger.kernel.org # v4.3+
Reported-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 include/linux/bio.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 5349e68..56d2db8 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -265,6 +265,26 @@ static inline unsigned bio_segments(struct bio *bio)
 	return segs;
 }
 
+static inline void bio_get_first_bvec(struct bio *bio, struct bio_vec *bv)
+{
+	*bv = bio_iovec(bio);
+}
+
+/*
+ * bio_get_last_bvec() is introduced to get the last bvec of one
+ * bio for bio_will_gap().
+ *
+ * TODO: make it more efficient.
+ */
+static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
+{
+	struct bvec_iter iter;
+
+	bio_for_each_segment(*bv, bio, iter)
+		if (bv->bv_len == iter.bi_size)
+			break;
+}
+
 /*
  * get a reference to a bio, so it won't disappear. the intended use is
  * something like:
-- 
1.9.1