[RFC PATCH v2 0/8] ext4: Add dma-buf read/write support

[Mengmeng Zhao <mengmeng.zhao@linux.dev>]

From: Mengmeng Zhao <zhaomengmeng@kylinos.cn>

This series adds dma-buf read/write support for ext4. The dma-buf direct
I/O infrastructure allows dma-buf to be handed to file I/O as an iov_iter,
but currently it only supports raw block devices. To better serve the
GPU-NVMe direct access through normal filesystems, prior work for XFS has
been discussed at [1]. We are also interested in this field and working on
it [2].

The series is based on v7.4-rc2 (7475ea8b5861), which already contains
the common dma-buf direct I/O infrastructure, so it no longer depends on a
separate base tree.
Patches 1-2 carry the block and iomap prerequisites from Nitesh Shetty and
Anuj Gupta:
- bdev_init_dma_buf_io_ctx in patch 1 was moved to block/bdev.c as
suggested by Christoph.
- iomap_file_init_dma_buf_io_ctx from that series is not carried. ext4
calls bdev_init_dma_buf_io_ctx directly, and the capability check is
handled by FMODE_CAN_DMABUF_IO.

The design is:
- FMODE_CAN_DMABUF_IO marks files that can be the target of dma-buf I/O;
  a filesystem sets it at open time for inodes whose data can be reached
  without the page cache.
- dma_buf_io_ctx_create() refuses files that lack the flag and checks the
  current f_flags for O_DIRECT at registration.
- As F_SETFL can clear O_DIRECT afterwards, the paths that would fall back
  to buffered or copied I/O still need per-I/O checks in the callers.
- DAX is rechecked per I/O because it can be enabled after open.
- iomap must not ask for a buffered fallback on behalf of a dma-buf
  iterator: -ENOTBLK would be refused by the callers and would turn a
  failure into a successful transfer of zero bytes.

Patch 3 keeps the buffered fallback out of dma-buf I/O, patches 4-7 add
the refusals on the common paths and the capability flag, and patch 8
enables ext4.

Known limitation: 
1. A dma-buf read that reaches a hole or an unwritten
extent fails as a whole, even when an earlier mapped extent was already
transferred into the buffer, so the caller does not learn how much of the
dma-buf was filled.  Reporting the prefix instead would return a short
read, which callers cannot tell apart from the end of the file, and both
the buffered and the direct path normally hide holes by zero-filling.
Probing the mapping before submitting anything would fix it, but that
belongs to the common iomap series.

2. A refused dma-buf I/O reaches the caller as -EOPNOTSUPP through
iomap_dio_set_error(), which also emits an fserror notification even
though no I/O was submitted.  The report could be limited to requests
that did submit a bio, since iomap_dio_bio_iter() accounts dio->size at
submission time, but that changes what fserror users see for every iomap
filesystem, so this version leaves it as it is and asks the fserror
maintainers.

Testing:
- io_uring dma-buf suite (udmabuf + REGISTER_BUFFERS_UPDATE + READ_FIXED /
  WRITE_FIXED) against ext4 on a PCIe NVMe namespace, all passed.
- xfstests -g quick on ext4 devices: 689 tests ran, 11 reported  failures,
  all of them environment related, no failure is attributable to this 
  series.

[1] https://lore.kernel.org/linux-xfs/20261016091919.971295-1-anuj1072538@gmail.com/
[2] https://lore.kernel.org/all/20260908131545.105987-1-liwang@kylinos.cn/

Changelog:
==========
RFC v2:
- Rebased onto v7.4-rc2 and dropped the dependency on a separate base
  tree; the block and iomap prerequisites are patches 1-2.
- Added patch 3, which reports why a dma-buf direct I/O could not be done
  instead of asking for a buffered fallback that the callers refuse.
- Split the block changes: patch 4 only factors the integrity check
  into a helper, and patch 5 refuses a dma-buf bio in the inline
  encryption fallback, which is where the data pages are walked
  instead of in blk_mq_submit_bio(). (Christoph)
- Dropped the mm/filemap and iomap_file_buffered_write() guards; the
  refusals now happen in the callers that would fall back to buffered
  or copied I/O. (Christoph)
- Added the FMODE_CAN_DMABUF_IO flag and use it to check whether a bdev or
  a filesystem can do dma-buf I/O. (Christoph)

RFC v1:
- https://lore.kernel.org/all/20261105065847.196540-1-mengmeng.zhao@linux.dev/

Mengmeng Zhao (6):
  iomap: don't ask for a buffered fallback for dma-buf iterators
  blk-mq: factor out a dma-buf bio refusal helper
  blk-crypto: refuse dma-buf bios in the fallback path
  fs: add FMODE_CAN_DMABUF_IO and require it for dma-buf I/O contexts
  dax: refuse dma-buf iterators in dax_iomap_rw()
  ext4: support dma-buf registered buffers for direct I/O

Nitesh Shetty (2):
  block: factor out bdev_init_dma_buf_io_ctx()
  iomap: add dma-buf direct I/O guards

 .../filesystems/iomap/operations.rst          |  3 ++
 block/bdev.c                                  | 14 ++++++
 block/blk-crypto.c                            |  8 ++++
 block/blk-mq.c                                | 24 ++++++----
 block/fops.c                                  | 12 +----
 drivers/dma-buf/dma-buf-io.c                  | 13 ++++--
 fs/dax.c                                      |  4 ++
 fs/ext4/ext4.h                                | 11 +++++
 fs/ext4/file.c                                | 44 +++++++++++++++++--
 fs/iomap/direct-io.c                          | 41 +++++++++++++++--
 include/linux/blkdev.h                        |  8 ++++
 include/linux/fs.h                            |  8 +++-
 12 files changed, 159 insertions(+), 31 deletions(-)


base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
-- 
2.43.0