Re: [RFC v2 04/11] block: introduce dma token backed bio type

[Christoph Hellwig <hch@infradead.org>]

> diff --git a/block/bio.c b/block/bio.c
> index 7b13bdf72de0..8793f1ee559d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -843,6 +843,11 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
>  		bio_clone_blkg_association(bio, bio_src);
>  	}
>  
> +	if (bio_flagged(bio_src, BIO_DMA_TOKEN)) {
> +		bio->dma_token = bio_src->dma_token;
> +		bio_set_flag(bio, BIO_DMA_TOKEN);
> +	}

Historically __bio_clone itself does not clone the payload, just the
bio.  But we got rid of the callers that want to clone a bio but not
the payload long time ago.

I'd suggest a prep patch that moves assigning bi_io_vec from
bio_alloc_clone and bio_init_clone into __bio_clone, and given that they
are the same field that'll take carw of the dma token as well.
Alternatively do it in an if/else that the compiler will hopefully
optimize away.

> @@ -1349,6 +1366,10 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter,
>  		bio_iov_bvec_set(bio, iter);
>  		iov_iter_advance(iter, bio->bi_iter.bi_size);
>  		return 0;
> +	} else if (iov_iter_is_dma_token(iter)) {

No else after an return please.

> +++ b/block/blk-merge.c
> @@ -328,6 +328,29 @@ int bio_split_io_at(struct bio *bio, const struct queue_limits *lim,
>  	unsigned nsegs = 0, bytes = 0, gaps = 0;
>  	struct bvec_iter iter;
>  
> +	if (bio_flagged(bio, BIO_DMA_TOKEN)) {

Please split the dmabuf logic into a self-contained
helper here.

> +		int offset = offset_in_page(bio->bi_iter.bi_bvec_done);
> +
> +		nsegs = ALIGN(bio->bi_iter.bi_size + offset, PAGE_SIZE);
> +		nsegs >>= PAGE_SHIFT;

Why are we hardcoding PAGE_SIZE based "segments" here?

> +
> +		if (offset & lim->dma_alignment || bytes & len_align_mask)
> +			return -EINVAL;
> +
> +		if (bio->bi_iter.bi_size > max_bytes) {
> +			bytes = max_bytes;
> +			nsegs = (bytes + offset) >> PAGE_SHIFT;
> +			goto split;
> +		} else if (nsegs > lim->max_segments) {

No else after a goto either.