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Re: [PATCH v3 02/10] iov_iter: add iterator type for dmabuf maps

by David Laight

On Wed, 29 Apr 2026 16:25:48 +0100 Pavel Begunkov <asml.silence(a)gmail.com> wrote: > Introduce a new iterator type for dmabuf maps. The map in an opaque > object with internals and format specific to the subsystem / driver, and > only it can use that subsystem / driver for issuing IO. The task of the > middle layers is to pass the map / iterator further down, maybe doing > basic splitting and length checking. The iterator can only be used by > operations of the file the associated map was created for. > > Suggested-by: Keith Busch <kbusch(a)kernel.org> > Signed-off-by: Pavel Begunkov <asml.silence(a)gmail.com> > --- > include/linux/uio.h | 11 +++++++++++ > lib/iov_iter.c | 29 +++++++++++++++++++++++------ > 2 files changed, 34 insertions(+), 6 deletions(-) > > diff --git a/include/linux/uio.h b/include/linux/uio.h > index a9bc5b3067e3..75051aed70de 100644 > --- a/include/linux/uio.h > +++ b/include/linux/uio.h > @@ -12,6 +12,7 @@ > > struct page; > struct folio_queue; > +struct io_dmabuf_map; > > typedef unsigned int __bitwise iov_iter_extraction_t; > > @@ -29,6 +30,7 @@ enum iter_type { > ITER_FOLIOQ, > ITER_XARRAY, > ITER_DISCARD, > + ITER_DMABUF_MAP, > }; > > #define ITER_SOURCE 1 // == WRITE > @@ -71,6 +73,7 @@ struct iov_iter { > const struct folio_queue *folioq; > struct xarray *xarray; > void __user *ubuf; > + struct io_dmabuf_map *dmabuf_map; > }; > size_t count; > }; > @@ -155,6 +158,11 @@ static inline bool iov_iter_is_xarray(const struct iov_iter *i) > return iov_iter_type(i) == ITER_XARRAY; > } > > +static inline bool iov_iter_is_dmabuf_map(const struct iov_iter *i) > +{ > + return iov_iter_type(i) == ITER_DMABUF_MAP; > +} > + > static inline unsigned char iov_iter_rw(const struct iov_iter *i) > { > return i->data_source ? WRITE : READ; > @@ -300,6 +308,9 @@ void iov_iter_folio_queue(struct iov_iter *i, unsigned int direction, > unsigned int first_slot, unsigned int offset, size_t count); > void iov_iter_xarray(struct iov_iter *i, unsigned int direction, struct xarray *xarray, > loff_t start, size_t count); > +void iov_iter_dmabuf_map(struct iov_iter *i, unsigned int direction, > + struct io_dmabuf_map *map, > + loff_t off, size_t count); > ssize_t iov_iter_get_pages2(struct iov_iter *i, struct page **pages, > size_t maxsize, unsigned maxpages, size_t *start); > ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i, struct page ***pages, > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > index 243662af1af7..e2253684b991 100644 > --- a/lib/iov_iter.c > +++ b/lib/iov_iter.c > @@ -575,7 +575,8 @@ void iov_iter_advance(struct iov_iter *i, size_t size) > { > if (unlikely(i->count < size)) > size = i->count; > - if (likely(iter_is_ubuf(i)) || unlikely(iov_iter_is_xarray(i))) { > + if (likely(iter_is_ubuf(i)) || unlikely(iov_iter_is_xarray(i)) || > + unlikely(iov_iter_is_dmabuf_map(i))) { Doesn't the extra check add more code to all the non-ubuf cases? This could be fixed by either making iter_type a bitmask (with one bit set) or writing an iter_is_one_of(i, ITER_xxx, ITER_yyy) define that uses '(1 << i->iter_type) & ((1 << ITER_xxx) | ...)' (look at the the nolibc printf code for an example). > i->iov_offset += size; > i->count -= size; > } else if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) { > @@ -631,7 +632,8 @@ void iov_iter_revert(struct iov_iter *i, size_t unroll) > return; > } > unroll -= i->iov_offset; > - if (iov_iter_is_xarray(i) || iter_is_ubuf(i)) { > + if (iov_iter_is_xarray(i) || iter_is_ubuf(i) || iter_is_ubuf() should have been first here. -- David > + iov_iter_is_dmabuf_map(i)) { > BUG(); /* We should never go beyond the start of the specified > * range since we might then be straying into pages that > * aren't pinned. > @@ -775,6 +777,20 @@ void iov_iter_xarray(struct iov_iter *i, unsigned int direction, > } > EXPORT_SYMBOL(iov_iter_xarray); > > +void iov_iter_dmabuf_map(struct iov_iter *i, unsigned int direction, > + struct io_dmabuf_map *map, > + loff_t off, size_t count) > +{ > + WARN_ON(direction & ~(READ | WRITE)); > + *i = (struct iov_iter){ > + .iter_type = ITER_DMABUF_MAP, > + .data_source = direction, > + .dmabuf_map = map, > + .count = count, > + .iov_offset = off, > + }; > +} > + > /** > * iov_iter_discard - Initialise an I/O iterator that discards data > * @i: The iterator to initialise. > @@ -841,7 +857,7 @@ static unsigned long iov_iter_alignment_bvec(const struct iov_iter *i) > > unsigned long iov_iter_alignment(const struct iov_iter *i) > { > - if (likely(iter_is_ubuf(i))) { > + if (likely(iter_is_ubuf(i)) || iov_iter_is_dmabuf_map(i)) { > size_t size = i->count; > if (size) > return ((unsigned long)i->ubuf + i->iov_offset) | size; > @@ -872,7 +888,7 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i) > size_t size = i->count; > unsigned k; > > - if (iter_is_ubuf(i)) > + if (iter_is_ubuf(i) || iov_iter_is_dmabuf_map(i)) > return 0; > > if (WARN_ON(!iter_is_iovec(i))) > @@ -1469,11 +1485,12 @@ EXPORT_SYMBOL_GPL(import_ubuf); > void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state) > { > if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i) && > - !iter_is_ubuf(i)) && !iov_iter_is_kvec(i)) > + !iter_is_ubuf(i) && !iov_iter_is_kvec(i) && > + !iov_iter_is_dmabuf_map(i))) > return; > i->iov_offset = state->iov_offset; > i->count = state->count; > - if (iter_is_ubuf(i)) > + if (iter_is_ubuf(i) || iov_iter_is_dmabuf_map(i)) > return; > /* > * For the *vec iters, nr_segs + iov is constant - if we increment

19 minutes

Re: [PATCH v2] drm/virtio: move cursor resv lock acquisition to prepare_fb

by Dmitry Osipenko

On 5/13/26 04:55, Deepanshu Kartikey wrote: > On Tue, May 12, 2026 at 12:04 PM Dmitry Osipenko > <dmitry.osipenko(a)collabora.com> wrote: >> >> I'm getting lockup with this patch applied and now see that >> virtio_gpu_resource_flush() also locks BO. >> >> Easiest option might be to add uninterruptible variant of >> virtio_gpu_array_lock_resv(). Could you please try it for v3? >> >> -- >> Best regards, >> Dmitry > > Hi Dmitry, > > Thanks for testing and catching the lockup. Before I send v3, want > to confirm the approach: > > 1. Revert v2's prepare_fb / cleanup_fb / plane_state changes; > keep the lock acquisition inside cursor_plane_update like > the original code. > > 2. Add virtio_gpu_array_lock_resv_uninterruptible() in > virtgpu_gem.c, mirroring the existing helper but using > dma_resv_lock() instead of dma_resv_lock_interruptible() on > the nents==1 path. Declare it in virtgpu_drv.h. > > 3. In cursor_plane_update, call the new helper and check its > return. The signal path is closed; -ENOMEM from > dma_resv_reserve_fences() remains and is handled by freeing > objs and skipping the cursor update for that frame. > > A skipped cursor frame on ENOMEM is the remaining failure mode in > .atomic_update; this avoids the lockup with virtio_gpu_resource_flush() > that v2's broader lock scope caused. > > Does that match what you had in mind? Sounds good. The virtio_gpu_resource_flush() also should be updated to use uninterruptible() variant. -- Best regards, Dmitry

1 hour, 15 minutes

Re: [PATCH v3 04/10] block: introduce dma map backed bio type

by Christoph Hellwig

> + union { > + struct bio_vec *bi_io_vec; > + /* Driver specific dma map, present only with BIO_DMABUF_MAP */ > + struct io_dmabuf_map *dmabuf_map; > + }; ... and please add the bi_ prefix we're using for all (well except for one oddity) fields in struct bio.

1 hour, 46 minutes

Re: [PATCH v3 07/10] nvme-pci: implement dma_token backed requests

by Christoph Hellwig

FYI, I really want SGL support before this get merged, but ignoring that for now: > +struct nvme_dmabuf_map { > + struct io_dmabuf_map base; > + dma_addr_t *dma_list; > + struct sg_table *sgt; > + unsigned nr_entries; I'd make dma_list a variable-sized array at the end of the struture to avoid an extra allocation and pointer derefernece. > > +static void nvme_dmabuf_map_sync(struct nvme_dev *nvme_dev, struct request *req, > + bool for_cpu) > +{ > + int length = blk_rq_payload_bytes(req); > + struct device *dev = nvme_dev->dev; > + enum dma_data_direction dma_dir; > + struct bio *bio = req->bio; > + struct nvme_dmabuf_map *map; > + dma_addr_t *dma_list; > + int offset, map_idx; > + > + dma_dir = rq_data_dir(req) == READ ? DMA_FROM_DEVICE : DMA_TO_DEVICE; > + map = container_of(bio->dmabuf_map, struct nvme_dmabuf_map, base); > + dma_list = map->dma_list; > + > + offset = bio->bi_iter.bi_bvec_done; > + map_idx = offset / NVME_CTRL_PAGE_SIZE; > + length += offset & (NVME_CTRL_PAGE_SIZE - 1); Please initialize the variable at declaration time and use or add proper helpers to simplify this: static inline struct nvme_dmabuf_map * to_nvme_dmabuf_map(struct io_dmabuf_map *map) { return container_of(map, struct nvme_dmabuf_map, base); } .... enum dma_data_direction dma_dir = rq_dma_dir(req); struct device *dev = nvme_dev->dev; struct bio *bio = req->bio; struct nvme_dmabuf_map *map = to_nvme_dmabuf_map(bio->bi_dmabuf_map); dma_addr_t *dma_list = map->dma_list; int offset = bio->bi_iter.bi_bvec_done; int mmap_idx = offset / NVME_CTRL_PAGE_SIZE; int length = blk_rq_payload_bytes(req) + offset & (NVME_CTRL_PAGE_SIZE - 1); Also a lot of these ints sound like they should be unsigned. > + > + while (length > 0) { > + u64 dma_addr = dma_list[map_idx++]; > + > + if (for_cpu) > + __dma_sync_single_for_cpu(dev, dma_addr, > + NVME_CTRL_PAGE_SIZE, dma_dir); > + else > + __dma_sync_single_for_device(dev, dma_addr, > + NVME_CTRL_PAGE_SIZE, > + dma_dir); > + length -= NVME_CTRL_PAGE_SIZE; > + } > +} Nothing should be using these __dma_sync helpers that are internal details. Using them means you call into sync code that should be skipped on most common server class systems. Also the for_cpu argument is a bit ugly. I'd rather have separate routines as in the core dma-mapping code, even if that means a little bit of code duplication. > +static blk_status_t nvme_rq_setup_dmabuf_map(struct request *req, > + struct nvme_queue *nvmeq) > +{ > + struct nvme_iod *iod = blk_mq_rq_to_pdu(req); > + int length = blk_rq_payload_bytes(req); > + u64 dma_addr, prp1_dma, prp2_dma; > + struct bio *bio = req->bio; > + struct nvme_dmabuf_map *map; > + dma_addr_t *dma_list; > + dma_addr_t prp_dma; > + __le64 *prp_list; > + int i, map_idx; > + int offset; > + > + nvme_dmabuf_map_sync(nvmeq->dev, req, false); > + > + map = container_of(bio->dmabuf_map, struct nvme_dmabuf_map, base); > + dma_list = map->dma_list; > + > + offset = bio->bi_iter.bi_bvec_done; > + map_idx = offset / NVME_CTRL_PAGE_SIZE; > + offset &= (NVME_CTRL_PAGE_SIZE - 1); > + prp1_dma = dma_list[map_idx++] + offset; Same comments as for the sync helper above. > + length -= (NVME_CTRL_PAGE_SIZE - offset); > + if (length <= 0) { > + prp2_dma = 0; > + goto done; > + } > + > + if (length <= NVME_CTRL_PAGE_SIZE) { > + prp2_dma = dma_list[map_idx]; > + goto done; > + } > + > + if (DIV_ROUND_UP(length, NVME_CTRL_PAGE_SIZE) <= > + NVME_SMALL_POOL_SIZE / sizeof(__le64)) > + iod->flags |= IOD_SMALL_DESCRIPTOR; > + > + prp_list = dma_pool_alloc(nvme_dma_pool(nvmeq, iod), GFP_ATOMIC, > + &prp_dma); > + if (!prp_list) > + return BLK_STS_RESOURCE; > + > + iod->descriptors[iod->nr_descriptors++] = prp_list; > + prp2_dma = prp_dma; And I really hate how this duplicates all the nasty PRP building logic, although right now I don't have a good answer to that. > +static inline bool nvme_rq_is_dmabuf_attached(struct request *req) > +{ > + if (!IS_ENABLED(CONFIG_DMABUF_TOKEN)) > + return false; > + return req->bio && bio_flagged(req->bio, BIO_DMABUF_MAP); > +} This is something that should go into the block layer.

1 hour, 47 minutes

Re: [PATCH v3 06/10] block: forward create_dmabuf_token to drivers

by Christoph Hellwig

On Wed, Apr 29, 2026 at 04:25:52PM +0100, Pavel Begunkov wrote: > Add a trivial implementation of the create_dmabuf_token call for > block devices that forwards the call to a new blk-mq callback if it's > available. This should go into block_device_operations as there is nothing blk-mq specific about this. I.e. even if this patchset doesn't handle stacking drivers yet, it should be easy enough to add them in the future.

1 hour, 59 minutes

Re: [PATCH v3 05/10] lib: add dmabuf token infrastructure

by Christoph Hellwig

Naming and placement: This is about dma-buf based I/O. So I'd expect it to be named dma-buf-io and no io-dmabuf, and live in drivers/dma-buf and not the unrelated lib/. But I'd like to hear from the dma-buf maintainers about that. Config option: as this unconditionally when DMA_SHARED_BUFFER is enabled, why does it need a separate config option? Interface: io_dmabuf_token_create / ->create_dmabuf_token filling in a structure allocated by the caller feels odd. My gut feeling would be to move most of io_dmabuf_token_create into a helper called by ->create_dmabuf_token so that the token is allocated in the driver data structure and returned from create_dmabuf_token.

2 hours, 1 minute

Re: [PATCH v3 04/10] block: introduce dma map backed bio type

by Christoph Hellwig

> + if (!bio_flagged(bio_src, BIO_DMABUF_MAP)) { > + bio->bi_io_vec = bio_src->bi_io_vec; > + } else { > + bio->dmabuf_map = bio_src->dmabuf_map; > + bio_set_flag(bio, BIO_DMABUF_MAP); > + } This is backwards, please avoid pointless negations: if (bio_flagged(bio_src, BIO_DMABUF_MAP)) { bio->dmabuf_map = bio_src->dmabuf_map; bio_set_flag(bio, BIO_DMABUF_MAP); } else { bio->bi_io_vec = bio_src->bi_io_vec; } > + if (bio_flagged(bio, BIO_DMABUF_MAP)) { > + nsegs = 1; > + > + if ((bio->bi_iter.bi_bvec_done & lim->dma_alignment) || > + (bio->bi_iter.bi_size & len_align_mask)) > + return -EINVAL; > + if (bio->bi_iter.bi_size > max_bytes) { > + bytes = max_bytes; > + goto split; > + } Please add a comment explaining why nsegs is always 1 here. > @@ -424,7 +424,8 @@ static inline struct bio *__bio_split_to_limits(struct bio *bio, > switch (bio_op(bio)) { > case REQ_OP_READ: > case REQ_OP_WRITE: > - if (bio_may_need_split(bio, lim)) > + if (bio_may_need_split(bio, lim) || > + bio_flagged(bio, BIO_DMABUF_MAP)) > return bio_split_rw(bio, lim, nr_segs); The BIO_DMABUF_MAP check should go into bio_may_need_split. > +static inline void bio_advance_iter_dmabuf_map(struct bvec_iter *iter, > + unsigned int bytes) > +{ > + iter->bi_bvec_done += bytes; > + iter->bi_size -= bytes; > +} > + > static inline void bio_advance_iter(const struct bio *bio, > struct bvec_iter *iter, unsigned int bytes) > { > iter->bi_sector += bytes >> 9; > > - if (bio_no_advance_iter(bio)) > + if (bio_no_advance_iter(bio)) { > iter->bi_size -= bytes; > - else > + } else if (bio_flagged(bio, BIO_DMABUF_MAP)) { > + bio_advance_iter_dmabuf_map(iter, bytes); This is a bit of a mess. You're using bi_bvec_done for something that is not bvec_done, which makes the naming very confusing. That is even more confusing than the existing usage, which isn't great. Also we add yet another conditional to heavily inlined code. I'd suggest the following: - add a prep patch to rename bi_bvec_done to bi_offset, as even for the existing usage it is the offset into the current bio_vec as much as it is the count of byes done, as those must be the same and it is used both ways - add a prep patch to also increase bi_offset for bio_no_advance_iter. It is not actually use there, but incrementing it is harmless and this will avoid a new special case - please also documet this new usage in the commet in struct bvec_iter. - then just add the dma buf mapping to the bio_no_advance_iter condition - figure out what to do about dm_bio_rewind_iter, which pokes into these things that really should be block layer internal > } > @@ -391,7 +403,7 @@ static inline void bio_wouldblock_error(struct bio *bio) > */ > static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs) > { > - if (iov_iter_is_bvec(iter)) > + if (iov_iter_is_bvec(iter) || iov_iter_is_dmabuf_map(iter)) > return 0; > return iov_iter_npages(iter, max_segs); > } Please update the comment for this helper. > @@ -322,6 +327,7 @@ enum { > BIO_REMAPPED, > BIO_ZONE_WRITE_PLUGGING, /* bio handled through zone write plugging */ > BIO_EMULATES_ZONE_APPEND, /* bio emulates a zone append operation */ > + BIO_DMABUF_MAP, /* Using premmaped dma buffers */ Shouldn't this be a REQ_ flag as we should never mix and match bios with and without this flag in a single request?

2 hours, 6 minutes

Re: [PATCH v3 03/10] block: move bvec init into __bio_clone

by Christoph Hellwig

On Wed, Apr 29, 2026 at 04:25:49PM +0100, Pavel Begunkov wrote: > To quote Cristoph: "Historically __bio_clone itself does not clone the It's Christoph. > payload, just the bio. But we got rid of the callers that want to clone > a bio but not the payload long time ago". So let's move ->bi_io_vec > assignment into __bio_clone(), so we have a single point where it's set. but this he-said, she-said is totally irrelevant. Please state here why this is useful/important, which matters for anyone trying to understand the code in the future.

2 hours, 13 minutes

Re: [PATCH v3 02/10] iov_iter: add iterator type for dmabuf maps

by Christoph Hellwig

On Wed, Apr 29, 2026 at 04:25:48PM +0100, Pavel Begunkov wrote: > Introduce a new iterator type for dmabuf maps. The map in an opaque > object with internals and format specific to the subsystem / driver, and > only it can use that subsystem / driver for issuing IO. The task of the > middle layers is to pass the map / iterator further down, maybe doing > basic splitting and length checking. The iterator can only be used by > operations of the file the associated map was created for. Nice, this ended up way simpler than what I would have imagined. Reviewed-by: Christoph Hellwig <hch(a)lst.de>

2 hours, 13 minutes

Re: [PATCH v3 01/10] file: add callback for creating long-term dmabuf maps

by Christoph Hellwig

On Thu, Apr 30, 2026 at 07:33:39PM +0100, Pavel Begunkov wrote: >> Then the patch should probably define the full interface and not just add the callback here and then the structure in a follow up patch. > > I strongly prefer splitting patches so that they touch one tree at > a time whenever possible. tbh, I don't see much of a problem it being > not defined as it's just forwarded in first patches, but I can shuffle > it around in the series so that definitions come first. file operations without users are pointless. This really should go with "lib: add dmabuf token infrastructure" as it is the only way for the reviewer to make any sense of it. I'll move my discussion of the interface there for that reason.

2 hours, 14 minutes

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