[PATCH V3 3/6] block: don't allocate inline bvecs if this bioset needn't bvecs

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

The inline bvecs won't be used if user needn't bvecs by not passing
BIOSET_NEED_BVECS, so don't allocate bvecs in this situation.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/bio.c         | 7 +++++--
 include/linux/bio.h | 1 +
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index cfa0e9db30e0..496aa5938f79 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -89,8 +89,7 @@ static struct bio_slab *create_bio_slab(unsigned int size)
 
 static inline unsigned int bs_bio_slab_size(struct bio_set *bs)
 {
-	return bs->front_pad + sizeof(struct bio) +
-		BIO_INLINE_VECS * sizeof(struct bio_vec);
+	return bs->front_pad + sizeof(struct bio) + bs->back_pad;
 }
 
 static struct kmem_cache *bio_find_or_create_slab(struct bio_set *bs)
@@ -1572,6 +1571,10 @@ int bioset_init(struct bio_set *bs,
 		int flags)
 {
 	bs->front_pad = front_pad;
+	if (flags & BIOSET_NEED_BVECS)
+		bs->back_pad = BIO_INLINE_VECS * sizeof(struct bio_vec);
+	else
+		bs->back_pad = 0;
 
 	spin_lock_init(&bs->rescue_lock);
 	bio_list_init(&bs->rescue_list);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 1edda614f7ce..f606eb1e556f 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -703,6 +703,7 @@ struct bio_set {
 	mempool_t bvec_integrity_pool;
 #endif
 
+	unsigned int back_pad;
 	/*
 	 * Deadlock avoidance for stacking block drivers: see comments in
 	 * bio_alloc_bioset() for details
-- 
2.28.0