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

Niklas Cassel
2025-12-03T06:13:02Z

On Tue, Dec 02, 2025 at 07:58:09PM -0800, Chaitanya Kulkarni wrote:
> 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>

Missing space before '<'.

Perhaps Jens can fix up when applying if you get no further comments.

Kind regards,
Niklas