Re: [PATCH v4 06/14] block: introduce dma map backed bio type

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/29/26 08:07, Christoph Hellwig wrote:
>> -	bio->bi_io_vec = bio_src->bi_io_vec;
>> +
>> +	if (op_is_dmabuf(bio->bi_opf)) {
>> +		bio->bi_dmabuf_map = bio_src->bi_dmabuf_map;
>> +	} else {
>> +		bio->bi_io_vec = bio_src->bi_io_vec;
>> +	}
> 
> No need for the braces.  

Got it,

But given that these are the fields why
> even bother doing both sides and not rely on the union?

Compiles into the same, but comparing to a comment makes
the compiler to check it for us if anything happens.

Noticed static_assert() below, that works as well, though I
think the current version is nicer b/c it preserves assignment
types for the compiler. Strict aliasing is obviously not a
thing, so don't have a strong opinion.

>> +void bio_dmabuf_map_set(struct bio *bio, struct iov_iter *iter)
>> +{
>> +	WARN_ON_ONCE(bio->bi_max_vecs);
>> +
>> +	bio->bi_dmabuf_map = iter->dmabuf_map;
>> +	bio->bi_vcnt = 0;
>> +	bio->bi_iter.bi_offset = iter->iov_offset;
>> +	bio->bi_iter.bi_size = iov_iter_count(iter);
>> +	bio->bi_opf |= REQ_NOMERGE | REQ_DMABUF;
> 
> This seems to be largely copied from bio_iov_bvec_set, but misses
> the REQ_CLONE there that we should probably set as well to indicate
> that the data descriptor is not owned (although not setting it is
> not really a bug).
> 
> What about merging these two into a single and easier to use interface
> like this:

No opinion on this, I can add it, probably as another prep patch.


> diff --git a/block/bio.c b/block/bio.c
> index cc2bb9183c1a..25393bdd119d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -860,12 +860,9 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
>   	bio->bi_write_hint = bio_src->bi_write_hint;
>   	bio->bi_write_stream = bio_src->bi_write_stream;
>   	bio->bi_iter = bio_src->bi_iter;
> -
> -	if (op_is_dmabuf(bio->bi_opf)) {
> -		bio->bi_dmabuf_map = bio_src->bi_dmabuf_map;
> -	} else {
> -		bio->bi_io_vec = bio_src->bi_io_vec;
> -	}
> +	static_assert(offsetof(struct bio, bi_io_vec) ==
> +		      offsetof(struct bio, bi_dmabuf_map));
> +	bio->bi_io_vec = bio_src->bi_io_vec;
>   
>   	if (bio->bi_bdev) {
>   		if (bio->bi_bdev == bio_src->bi_bdev &&
> @@ -1186,26 +1183,21 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>   }
>   EXPORT_SYMBOL_GPL(__bio_release_pages);
>   
> -void bio_iov_bvec_set(struct bio *bio, const struct iov_iter *iter)
> +bool bio_iov_iter_set(struct bio *bio, const struct iov_iter *iter)
>   {
>   	WARN_ON_ONCE(bio->bi_max_vecs);
>   
> +	if (!iov_iter_is_bvec(iter) && !iov_iter_is_dmabuf_map(iter))
> +		return false;
> +
>   	bio->bi_io_vec = (struct bio_vec *)iter->bvec;
>   	bio->bi_iter.bi_idx = 0;
>   	bio->bi_iter.bi_offset = iter->iov_offset;
>   	bio->bi_iter.bi_size = iov_iter_count(iter);
>   	bio_set_flag(bio, BIO_CLONED);
> -}
> -
> -void bio_dmabuf_map_set(struct bio *bio, struct iov_iter *iter)
> -{
> -	WARN_ON_ONCE(bio->bi_max_vecs);
> -
> -	bio->bi_dmabuf_map = iter->dmabuf_map;
> -	bio->bi_vcnt = 0;
> -	bio->bi_iter.bi_offset = iter->iov_offset;
> -	bio->bi_iter.bi_size = iov_iter_count(iter);
> -	bio->bi_opf |= REQ_NOMERGE | REQ_DMABUF;
> +	if (iov_iter_is_dmabuf_map(iter))
> +		bio->bi_opf |= REQ_NOMERGE | REQ_DMABUF;
> +	return true;
>   }
>   
>   /*
> @@ -1265,13 +1257,7 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter,
>   	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
>   		return -EIO;
>   
> -	if (iov_iter_is_bvec(iter)) {
> -		bio_iov_bvec_set(bio, iter);
> -		iov_iter_advance(iter, bio->bi_iter.bi_size);
> -		return 0;
> -	}
> -	if (iov_iter_is_dmabuf_map(iter)) {
> -		bio_dmabuf_map_set(bio, iter);
> +	if (bio_iov_iter_set(bio, iter)) {
>   		iov_iter_advance(iter, bio->bi_iter.bi_size);
>   		return 0;
>   	}
> diff --git a/block/blk-map.c b/block/blk-map.c
> index d1d6bbe0ecf1..34816e6e64de 100644
> --- a/block/blk-map.c
> +++ b/block/blk-map.c
> @@ -473,7 +473,7 @@ static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter)
>   	bio = blk_rq_map_bio_alloc(rq, 0, GFP_KERNEL);
>   	if (!bio)
>   		return -ENOMEM;
> -	bio_iov_bvec_set(bio, iter);
> +	bio_iov_iter_set(bio, iter);
>   
>   	ret = blk_rq_append_bio(rq, bio);
>   	if (ret)
> diff --git a/block/fops.c b/block/fops.c
> index d83cdbab65a6..97fb124a19b2 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -340,17 +340,12 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
>   	bio->bi_end_io = blkdev_bio_end_io_async;
>   	bio->bi_ioprio = iocb->ki_ioprio;
>   
> -	if (iov_iter_is_bvec(iter)) {
> -		/*
> -		 * Users don't rely on the iterator being in any particular
> -		 * state for async I/O returning -EIOCBQUEUED, hence we can
> -		 * avoid expensive iov_iter_advance(). Bypass
> -		 * bio_iov_iter_get_pages() and set the bvec directly.
> -		 */
> -		bio_iov_bvec_set(bio, iter);
> -	} else if (iov_iter_is_dmabuf_map(iter)) {
> -		bio_dmabuf_map_set(bio, iter);
> -	} else {
> +	/*
> +	 * Users don't rely on the iterator being in any particular state for
> +	 * async I/O returning -EIOCBQUEUED, hence we can avoid the expensive
> +	 * iov_iter_advance() if we could set the bvec/dmabuf directly.
> +	 */
> +	if (!bio_iov_iter_set(bio, iter)) {
>   		ret = blkdev_iov_iter_get_pages(bio, iter, bdev);
>   		if (unlikely(ret))
>   			goto out_bio_put;
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index c053e9444514..1eff02a843ef 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -484,8 +484,7 @@ int bdev_rw_virt(struct block_device *bdev, sector_t sector, void *data,
>   int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter,
>   		unsigned len_align_mask);
>   
> -void bio_iov_bvec_set(struct bio *bio, const struct iov_iter *iter);
> -void bio_dmabuf_map_set(struct bio *bio, struct iov_iter *iter);
> +bool bio_iov_iter_set(struct bio *bio, const struct iov_iter *iter);
>   void __bio_release_pages(struct bio *bio, bool mark_dirty);
>   extern void bio_set_pages_dirty(struct bio *bio);
>   extern void bio_check_pages_dirty(struct bio *bio);

-- 
Pavel Begunkov