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 --- drivers/block/loop.c | 5 ++--- drivers/block/zloop.c | 5 ++--- 2 files changed, 4 insertions(+), 6 deletions(-) (limited to 'drivers/block') diff --git a/drivers/block/loop.c b/drivers/block/loop.c index ebe751f39742..272bc608e528 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -348,11 +348,10 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd, struct file *file = lo->lo_backing_file; struct bio_vec tmp; unsigned int offset; - int nr_bvec = 0; + unsigned int nr_bvec; int ret; - rq_for_each_bvec(tmp, rq, rq_iter) - nr_bvec++; + nr_bvec = blk_rq_nr_bvec(rq); if (rq->bio != rq->biotail) { diff --git a/drivers/block/zloop.c b/drivers/block/zloop.c index 3f50321aa4a7..77bd6081b244 100644 --- a/drivers/block/zloop.c +++ b/drivers/block/zloop.c @@ -394,7 +394,7 @@ static void zloop_rw(struct zloop_cmd *cmd) struct bio_vec tmp; unsigned long flags; sector_t zone_end; - int nr_bvec = 0; + unsigned int nr_bvec; int ret; atomic_set(&cmd->ref, 2); @@ -487,8 +487,7 @@ static void zloop_rw(struct zloop_cmd *cmd) spin_unlock_irqrestore(&zone->wp_lock, flags); } - rq_for_each_bvec(tmp, rq, rq_iter) - nr_bvec++; + nr_bvec = blk_rq_nr_bvec(rq); if (rq->bio != rq->biotail) { struct bio_vec *bvec; -- cgit