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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 <cassel@kernel.org>
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
master
Chaitanya Kulkarni 2025-12-02 19:58:09 -08:00 committed by Jens Axboe
parent 3e2cb9ee76
commit 71075d25ca
3 changed files with 22 additions and 6 deletions
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5
drivers/block/loop.c
View File

@ -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) {

5
drivers/block/zloop.c
View File

@ -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;

18
include/linux/blk-mq.h
View File

@ -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)