bio: add support for inlining a number of bio_vecs inside the bio

When we go and allocate a bio for IO, we actually do two allocations.
One for the bio itself, and one for the bi_io_vec that holds the
actual pages we are interested in.

This feature inlines a definable amount of io vecs inside the bio
itself, so we eliminate the bio_vec array allocation for IO's up
to a certain size. It defaults to 4 vecs, which is typically 16k
of IO.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/bio.h

@@ -102,6 +102,13 @@ struct bio {
 #endif
 
 	bio_destructor_t	*bi_destructor;	/* destructor */
+
+	/*
+	 * We can inline a number of vecs at the end of the bio, to avoid
+	 * double allocations for a small number of bio_vecs. This member
+	 * MUST obviously be kept at the very end of the bio.
+	 */
+	struct bio_vec		bi_inline_vecs[0];
 };
 
 /*
@@ -213,6 +220,11 @@ static inline void *bio_data(struct bio *bio)
 	return NULL;
 }
 
+static inline int bio_has_allocated_vec(struct bio *bio)
+{
+	return bio->bi_io_vec && bio->bi_io_vec != bio->bi_inline_vecs;
+}
+
 /*
  * will die
  */