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([2620:10d:c092:600::1:640]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c1f83c6858asm99322666b.10.2026.07.29.04.03.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Jul 2026 04:03:56 -0700 (PDT) Message-ID: Date: Wed, 29 Jul 2026 12:03:46 +0100 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 06/14] block: introduce dma map backed bio type To: Christoph Hellwig Cc: Jens Axboe , Keith Busch , Sagi Grimberg , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, Alexander Viro , Christian Brauner , Andrew Morton , Sumit Semwal , =?UTF-8?Q?Christian_K=C3=B6nig?= , Nitesh Shetty , Kanchan Joshi , Anuj Gupta , Tushar Gohad , William Power , Phil Cayton , Jason Gunthorpe , Damien Le Moal , Alasdair Kergon , Mike Snitzer , Mikulas Patocka , Benjamin Marzinski , Vishal Verma , David Sterba , Ilya Dryomov , dm-devel@lists.linux.dev, nvdimm@lists.linux.dev, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org References: <20260729070727.GF9534@lst.de> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20260729070727.GF9534@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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