[PATCH V3] blk-mq: add blk_rq_nr_bvec() helper

Chaitanya Kulkarni
2025-12-03T03:58:12Z

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>
---

V3:-
1. Rebased and Added Reviewed-by tag. (Niklas)

Hi,

During bio submission, the block layer may split a single bvec into
multiple physical segments based on device limits:
submit_bio()
-> submit_bio_noacct()
-> __submit_bio_noacct()
-> __submit_bio()
-> blk_mq_submit_bio()
-> __bio_split_to_limits()
-> bio_split_rw()
-> bio_split_rw_at()
-> bio_split_io_at()
-> bio_for_each_bvec()
-> [Fast path] nsegs++ (1 bvec = 1 segment)
-> [Slow path] bvec_split_segs()

The bvec_split_segs() function handles the case where a single bvec must
be split into multiple segments:

/**
* bvec_split_segs - verify whether or not a bvec should be split in the
* middle
* ...
* When splitting a bio, it can happen that a bvec is encountered that is
* too big to fit in a single segment and hence that it has to be split in
* the middle.
*/
static bool bvec_split_segs(...)
{
while (len && *nsegs < max_segs) {
seg_size = get_max_segment_size(...);
(*nsegs)++;
total_len += seg_size;
len -= seg_size;
}
*bytes += total_len;
return (len > 0);
}

Splitting occurs when a bvec exceeds:
- max_segment_size
- segment_boundary_mask (DMA boundary constraints)
- max_segments limit

Result after bio_split_io_at():
- nr_bvec (what rq_for_each_bvec iterates): **1**
- rq->nr_phys_segments: 2

*[ 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*

-ck
---
drivers/block/loop.c | 5 ++---
drivers/block/zloop.c | 5 ++---
include/linux/blk-mq.h | 18 ++++++++++++++++++
3 files changed, 22 insertions(+), 6 deletions(-)

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;
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)
--
2.40.0