Traditionally, the conditions for when DIO (direct I/O) is supported
were fairly simple.  For both block devices and regular files, DIO had
to be aligned to the logical block size of the block device.

However, due to filesystem features that have been added over time (e.g.
multi-device support, data journalling, inline data, encryption, verity,
compression, checkpoint disabling, log-structured mode), the conditions
for when DIO is allowed on a regular file have gotten increasingly
complex.  Whether a particular regular file supports DIO, and with what
alignment, can depend on various file attributes and filesystem mount
options, as well as which block device(s) the file's data is located on.

Moreover, the general rule of DIO needing to be aligned to the block
device's logical block size was recently relaxed to allow user buffers
(but not file offsets) aligned to the DMA alignment instead.  See
commit bf8d0853 ("iomap: add support for dma aligned direct-io").

XFS has an ioctl XFS_IOC_DIOINFO that exposes DIO alignment information.
Uplifting this to the VFS is one possibility.  However, as discussed
(https://lore.kernel.org/linux-fsdevel/20220120071215.123274-1-ebiggers@kernel.org/T/#u),
this ioctl is rarely used and not known to be used outside of
XFS-specific code.  It was also never intended to indicate when a file
doesn't support DIO at all, nor was it intended for block devices.

Therefore, let's expose this information via statx().  Add the
STATX_DIOALIGN flag and two new statx fields associated with it:

* stx_dio_mem_align: the alignment (in bytes) required for user memory
  buffers for DIO, or 0 if DIO is not supported on the file.

* stx_dio_offset_align: the alignment (in bytes) required for file
  offsets and I/O segment lengths for DIO, or 0 if DIO is not supported
  on the file.  This will only be nonzero if stx_dio_mem_align is
  nonzero, and vice versa.

Note that as with other statx() extensions, if STATX_DIOALIGN isn't set
in the returned statx struct, then these new fields won't be filled in.
This will happen if the file is neither a regular file nor a block
device, or if the file is a regular file and the filesystem doesn't
support STATX_DIOALIGN.  It might also happen if the caller didn't
include STATX_DIOALIGN in the request mask, since statx() isn't required
to return unrequested information.

This commit only adds the VFS-level plumbing for STATX_DIOALIGN.  For
regular files, individual filesystems will still need to add code to
support it.  For block devices, a separate commit will wire it up too.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NMartin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: NChristian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220827065851.135710-2-ebiggers@kernel.org