From 71075d25ca5cae732fb57da065fbf14aeb3bcfc7 Mon Sep 17 00:00:00 2001 From: Chaitanya Kulkarni Date: Tue, 2 Dec 2025 19:58:09 -0800 Subject: blk-mq: add blk_rq_nr_bvec() helper Add a new helper function blk_rq_nr_bvec() that returns the number of bvecs in a request. This count represents the number of iterations rq_for_each_bvec() would perform on a request. Drivers need to pre-allocate bvec arrays before iterating through a request's bvecs. Currently, they manually count bvecs using rq_for_each_bvec() in a loop, which is repetitive. The new helper centralizes this logic. This pattern exists in loop and zloop drivers, where multi-bio requests require copying bvecs into a contiguous array before creating an iov_iter for file operations. Update loop and zloop drivers to use the new helper, eliminating duplicate code. This patch also provides a clear API to avoid any potential misuse of blk_nr_phys_segments() for calculating the bvecs since, one bvec can have more than one segments and use of blk_nr_phys_segments() can lead to extra memory allocation :- [ 6155.673749] nullb_bio: 128K bio as ONE bvec: sector=0, size=131072 [ 6155.673846] null_blk: #### null_handle_data_transfer:1375 [ 6155.673850] null_blk: nr_bvec=1 blk_rq_nr_phys_segments=2 [ 6155.674263] null_blk: #### null_handle_data_transfer:1375 [ 6155.674267] null_blk: nr_bvec=1 blk_rq_nr_phys_segments=1 Reviewed-by: Niklas Cassel Signed-off-by: Chaitanya Kulkarni Reviewed-by: Christoph Hellwig Signed-off-by: Jens Axboe --- include/linux/blk-mq.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'include') diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index eb7254b3dddd..cae9e857aea4 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -1213,6 +1213,24 @@ static inline unsigned short blk_rq_nr_discard_segments(struct request *rq) return max_t(unsigned short, rq->nr_phys_segments, 1); } +/** + * blk_rq_nr_bvec - return number of bvecs in a request + * @rq: request to calculate bvecs for + * + * Returns the number of bvecs. + */ +static inline unsigned int blk_rq_nr_bvec(struct request *rq) +{ + struct req_iterator rq_iter; + struct bio_vec bv; + unsigned int nr_bvec = 0; + + rq_for_each_bvec(bv, rq, rq_iter) + nr_bvec++; + + return nr_bvec; +} + int __blk_rq_map_sg(struct request *rq, struct scatterlist *sglist, struct scatterlist **last_sg); static inline int blk_rq_map_sg(struct request *rq, struct scatterlist *sglist) -- cgit v1.2.3