Re: [PATCH V2] blk-mq: add blk_rq_nr_bvec() helper

[Niklas Cassel <cassel@kernel.org>]

On Tue, Nov 11, 2025 at 03:22:52PM -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
> 
> Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
> ---

What is the status of this patch?


FWIW, looks good to me:
Reviewed-by: Niklas Cassel <cassel@kernel.org>