Re: [PATCH for-next v10 6/7] block: extend functionality to map bvec iterator

Anuj Gupta
2022-09-29T11:50:33Z

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On Wed, Sep 28, 2022 at 07:40:39PM +0200, Christoph Hellwig wrote:
> On Tue, Sep 27, 2022 at 11:06:09PM +0530, Kanchan Joshi wrote:
> > Extend blk_rq_map_user_iov so that it can handle bvec iterator.
> > It maps the pages from bvec iterator into a bio and place the bio into
> > request.
> >
> > This helper will be used by nvme for uring-passthrough path when IO is
> > done using pre-mapped buffers.
>
> Can we avoid duplicating some of the checks? Something like the below
> incremental patch. Note that this now also allows the copy path for
> all kinds of iov_iters, but as the copy from/to iter code is safe
> and the sanity check was just or the map path that should be fine.
> It's best split into a prep patch, though.

Right, this one looks way better. Will fold this in a separate prep patch.

>
> ---
> diff --git a/block/blk-map.c b/block/blk-map.c
> index a1aa8dacb02bc..c51de30767403 100644
> --- a/block/blk-map.c
> +++ b/block/blk-map.c
> @@ -549,26 +549,16 @@ int blk_rq_append_bio(struct request *rq, struct bio *bio)
> EXPORT_SYMBOL(blk_rq_append_bio);
>
> /* Prepare bio for passthrough IO given ITER_BVEC iter */
> -static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter,
> - bool *copy)
> +static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter)
> {
> struct request_queue *q = rq->q;
> - size_t nr_iter, nr_segs, i;
> - struct bio *bio = NULL;
> - struct bio_vec *bv, *bvecs, *bvprvp = NULL;
> + size_t nr_iter = iov_iter_count(iter);
> + size_t nr_segs = iter->nr_segs;
> + struct bio_vec *bvecs, *bvprvp = NULL;
> struct queue_limits *lim = &q->limits;
> unsigned int nsegs = 0, bytes = 0;
> - unsigned long align = q->dma_pad_mask | queue_dma_alignment(q);
> -
> - /* see if we need to copy pages due to any weird situation */
> - if (blk_queue_may_bounce(q))
> - goto out_copy;
> - else if (iov_iter_alignment(iter) & align)
> - goto out_copy;
> - /* virt-alignment gap is checked anyway down, so avoid extra loop here */
> -
> - nr_iter = iov_iter_count(iter);
> - nr_segs = iter->nr_segs;
> + struct bio *bio;
> + size_t i;
>
> if (!nr_iter || (nr_iter >> SECTOR_SHIFT) > queue_max_hw_sectors(q))
> return -EINVAL;
> @@ -586,14 +576,15 @@ static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter,
> /* loop to perform a bunch of sanity checks */
> bvecs = (struct bio_vec *)iter->bvec;
> for (i = 0; i < nr_segs; i++) {
> - bv = &bvecs[i];
> + struct bio_vec *bv = &bvecs[i];
> +
> /*
> * If the queue doesn't support SG gaps and adding this
> * offset would create a gap, fallback to copy.
> */
> if (bvprvp && bvec_gap_to_prev(lim, bvprvp, bv->bv_offset)) {
> bio_map_put(bio);
> - goto out_copy;
> + return -EREMOTEIO;
> }
> /* check full condition */
> if (nsegs >= nr_segs || bytes > UINT_MAX - bv->bv_len)
> @@ -611,9 +602,6 @@ static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter,
> put_bio:
> bio_map_put(bio);
> return -EINVAL;
> -out_copy:
> - *copy = true;
> - return 0;
> }
>
> /**
> @@ -635,33 +623,35 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
> struct rq_map_data *map_data,
> const struct iov_iter *iter, gfp_t gfp_mask)
> {
> - bool copy = false;
> + bool copy = false, map_bvec = false;
> unsigned long align = q->dma_pad_mask | queue_dma_alignment(q);
> struct bio *bio = NULL;
> struct iov_iter i;
> int ret = -EINVAL;
>
> - if (iov_iter_is_bvec(iter)) {
> - ret = blk_rq_map_user_bvec(rq, iter, &copy);
> - if (ret != 0)
> - goto fail;
> - if (copy)
> - goto do_copy;
> - return ret;
> - }
> - if (!iter_is_iovec(iter))
> - goto fail;
> -
> if (map_data)
> copy = true;
> else if (blk_queue_may_bounce(q))
> copy = true;
> else if (iov_iter_alignment(iter) & align)
> copy = true;
> + else if (iov_iter_is_bvec(iter))
> + map_bvec = true;
> + else if (!iter_is_iovec(iter))
> + copy = true;
> else if (queue_virt_boundary(q))
> copy = queue_virt_boundary(q) & iov_iter_gap_alignment(iter);
>
> -do_copy:
> + if (map_bvec) {
> + ret = blk_rq_map_user_bvec(rq, iter);
> + if (!ret)
> + return 0;
> + if (ret != -EREMOTEIO)
> + goto fail;
> + /* fall back to copying the data on limits mismatches */
> + copy = true;
> + }
> +
> i = *iter;
> do {
> if (copy)
>

--
Anuj Gupta

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