[PATCH 06/13] rbd: use bio_clone_fast() instead of bio_clone()

NeilBrown
2017-06-18T04:40:13Z

bio_clone() makes a copy of the bi_io_vec, but rbd never changes that,
so there is no need for a copy.
bio_clone_fast() can be used instead, which avoids making the copy.

This requires that we provide a bio_set. bio_clone() uses fs_bio_set,
but it isn't, in general, safe to use the same bio_set at different
levels of the stack, as that can lead to deadlocks. As filesystems
use fs_bio_set, block devices shouldn't.

As rbd never stacks, it is safe to have a single global bio_set for
all rbd devices to use. So allocate that when the module is
initialised, and use it with bio_clone_fast().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/block/rbd.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 5420bc40c544..b008b6a98098 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -442,6 +442,8 @@ static DEFINE_SPINLOCK(rbd_client_list_lock);
static struct kmem_cache *rbd_img_request_cache;
static struct kmem_cache *rbd_obj_request_cache;

+static struct bio_set *rbd_bio_clone;
+
static int rbd_major;
static DEFINE_IDA(rbd_dev_id_ida);

@@ -1363,7 +1365,7 @@ static struct bio *bio_clone_range(struct bio *bio_src,
{
struct bio *bio;

- bio = bio_clone(bio_src, gfpmask);
+ bio = bio_clone_fast(bio_src, gfpmask, rbd_bio_clone);
if (!bio)
return NULL; /* ENOMEM */

@@ -6416,8 +6418,16 @@ static int rbd_slab_init(void)
if (!rbd_obj_request_cache)
goto out_err;

+ rbd_assert(!rbd_bio_clone);
+ rbd_bio_clone = bioset_create(BIO_POOL_SIZE, 0, 0);
+ if (!rbd_bio_clone)
+ goto out_err_clone;
+
return 0;

+out_err_clone:
+ kmem_cache_destroy(rbd_obj_request_cache);
+ rbd_obj_request_cache = NULL;
out_err:
kmem_cache_destroy(rbd_img_request_cache);
rbd_img_request_cache = NULL;
@@ -6433,6 +6443,10 @@ static void rbd_slab_exit(void)
rbd_assert(rbd_img_request_cache);
kmem_cache_destroy(rbd_img_request_cache);
rbd_img_request_cache = NULL;
+
+ rbd_assert(rbd_bio_clone);
+ bioset_free(rbd_bio_clone);
+ rbd_bio_clone = NULL;
}

static int __init rbd_init(void)