Re: [PATCH for-next v5 3/4] block: add helper to map bvec iterator for passthrough

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 9/5/22 23:27, Kanchan Joshi wrote:
> Add blk_rq_map_user_bvec which maps the bvec iterator into a bio and
> places that into the request. This helper will be used in nvme for
> uring-passthrough with fixed-buffer.
> While at it, create another helper bio_map_get to reduce the code
> duplication.
> 
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> ---
>   block/blk-map.c        | 94 +++++++++++++++++++++++++++++++++++++-----
>   include/linux/blk-mq.h |  1 +
>   2 files changed, 85 insertions(+), 10 deletions(-)
> 
> diff --git a/block/blk-map.c b/block/blk-map.c
> index f3768876d618..e2f268167342 100644
> --- a/block/blk-map.c
> +++ b/block/blk-map.c
> @@ -241,17 +241,10 @@ static void bio_map_put(struct bio *bio)
>   	}
>   }
>   
> -static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
> +static struct bio *bio_map_get(struct request *rq, unsigned int nr_vecs,
>   		gfp_t gfp_mask)
>   {
> -	unsigned int max_sectors = queue_max_hw_sectors(rq->q);
> -	unsigned int nr_vecs = iov_iter_npages(iter, BIO_MAX_VECS);
>   	struct bio *bio;
> -	int ret;
> -	int j;
> -
> -	if (!iov_iter_count(iter))
> -		return -EINVAL;
>   
>   	if (rq->cmd_flags & REQ_POLLED) {
>   		blk_opf_t opf = rq->cmd_flags | REQ_ALLOC_CACHE;
> @@ -259,13 +252,31 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
>   		bio = bio_alloc_bioset(NULL, nr_vecs, opf, gfp_mask,
>   					&fs_bio_set);
>   		if (!bio)
> -			return -ENOMEM;
> +			return NULL;
>   	} else {
>   		bio = bio_kmalloc(nr_vecs, gfp_mask);
>   		if (!bio)
> -			return -ENOMEM;
> +			return NULL;
>   		bio_init(bio, NULL, bio->bi_inline_vecs, nr_vecs, req_op(rq));
>   	}
> +	return bio;
> +}
> +
> +static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
> +		gfp_t gfp_mask)
> +{
> +	unsigned int max_sectors = queue_max_hw_sectors(rq->q);
> +	unsigned int nr_vecs = iov_iter_npages(iter, BIO_MAX_VECS);
> +	struct bio *bio;
> +	int ret;
> +	int j;
> +
> +	if (!iov_iter_count(iter))
> +		return -EINVAL;
> +
> +	bio = bio_map_get(rq, nr_vecs, gfp_mask);
> +	if (bio == NULL)
> +		return -ENOMEM;
>   
>   	while (iov_iter_count(iter)) {
>   		struct page **pages, *stack_pages[UIO_FASTIOV];
> @@ -612,6 +623,69 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
>   }
>   EXPORT_SYMBOL(blk_rq_map_user);
>   
> +/* Prepare bio for passthrough IO given an existing bvec iter */
> +int blk_rq_map_user_bvec(struct request *rq, struct iov_iter *iter)
> +{
> +	struct request_queue *q = rq->q;
> +	size_t iter_count, nr_segs;
> +	struct bio *bio;
> +	struct bio_vec *bv, *bvec_arr, *bvprvp = NULL;
> +	struct queue_limits *lim = &q->limits;
> +	unsigned int nsegs = 0, bytes = 0;
> +	int ret, i;
> +

consider this (untested), it also sets the variable i data type same
as it comparison variable in nr_segs the loop i.e. size_t :-

+       struct bio_vec *bv, *bvec_arr, *bvprvp = NULL;
+       struct request_queue *q = rq->q;
+       struct queue_limits *lim = &q->limits;
+       unsigned int nsegs = 0, bytes = 0;
+       size_t iter_count, nr_segs, i;
+       struct bio *bio;
+       int ret;


> +	iter_count = iov_iter_count(iter);
> +	nr_segs = iter->nr_segs;
> +
> +	if (!iter_count || (iter_count >> 9) > queue_max_hw_sectors(q))
> +		return -EINVAL;

can we remove braces for iter_count >> 9 without impacting the intended
functionality?

> +	if (nr_segs > queue_max_segments(q))
> +		return -EINVAL;
> +
> +	/* no iovecs to alloc, as we already have a BVEC iterator */
> +	bio = bio_map_get(rq, 0, GFP_KERNEL);
> +	if (bio == NULL)
> +		return -ENOMEM;
> +
> +	bio_iov_bvec_set(bio, iter);
> +	blk_rq_bio_prep(rq, bio, nr_segs);
> +
> +	/* loop to perform a bunch of sanity checks */
> +	bvec_arr = (struct bio_vec *)iter->bvec;
> +	for (i = 0; i < nr_segs; i++) {
> +		bv = &bvec_arr[i];

I'd just call bvecs instead of bvec_arr, just personal preference.

> +		/*
> +		 * If the queue doesn't support SG gaps and adding this
> +		 * offset would create a gap, disallow it.
> +		 */
> +		if (bvprvp && bvec_gap_to_prev(lim, bvprvp, bv->bv_offset)) {
> +			ret = -EINVAL;
> +			goto out_free;
> +		}
> +
> +		/* check full condition */
> +		if (nsegs >= nr_segs || bytes > UINT_MAX - bv->bv_len) {
> +			ret = -EINVAL;
> +			goto out_free;
> +		}
> +
> +		if (bytes + bv->bv_len <= iter_count &&
> +				bv->bv_offset + bv->bv_len <= PAGE_SIZE) {
> +			nsegs++;
> +			bytes += bv->bv_len;
> +		} else {
> +			ret = -EINVAL;
> +			goto out_free;

you are only calling goto out_free; from loop with ret = -EINVAL.
you can remove redundant ret = -EINVAL assignment in the loop and
call return -EINVAL from the out_free: label instead return ret.
That will also allow us to remove braces in the loop.

This will also allow us to remove the ret variable on the
since I guess we are in the fast path.

> +		}
> +		bvprvp = bv;
> +	}
> +	return 0;
> +out_free:
> +	bio_map_put(bio);
> +	return ret;
> +}
> +EXPORT_SYMBOL(blk_rq_map_user_bvec);

why not use EXPORT_SYMBOL_GPL() for new addition?

I'm aware that blk-map.c only uses EXPORT_SYMBOL().

> +
>   /**
>    * blk_rq_unmap_user - unmap a request with user data
>    * @bio:	       start of bio list
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index b43c81d91892..83bef362f0f9 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -970,6 +970,7 @@ struct rq_map_data {
>   	bool from_user;
>   };
>   
> +int blk_rq_map_user_bvec(struct request *rq, struct iov_iter *iter);
>   int blk_rq_map_user(struct request_queue *, struct request *,
>   		struct rq_map_data *, void __user *, unsigned long, gfp_t);
>   int blk_rq_map_user_iov(struct request_queue *, struct request *,