xfs: add xfs_file_dio_write_atomic()

Add xfs_file_dio_write_atomic() for dedicated handling of atomic writes.

Now HW offload will not be required to support atomic writes and is
an optional feature.

CoW-based atomic writes will be supported with out-of-places write and
atomic extent remapping.

Either mode of operation may be used for an atomic write, depending on the
circumstances.

The preferred method is HW offload as it will be faster. If HW offload is
not available then we always use the CoW-based method.  If HW offload is
available but not possible to use, then again we use the CoW-based method.

If available, HW offload would not be possible for the write length
exceeding the HW offload limit, the write spanning multiple extents,
unaligned disk blocks, etc.

Apart from the write exceeding the HW offload limit, other conditions for
HW offload usage can only be detected in the iomap handling for the write.
As such, we use a fallback method to issue the write if we detect in the
->iomap_begin() handler that HW offload is not possible. Special code
-ENOPROTOOPT is returned from ->iomap_begin() to inform that HW offload is
not possible.

In other words, atomic writes are supported on any filesystem that can
perform out of place write remapping atomically (i.e. reflink) up to
some fairly large size.  If the conditions are right (a single correctly
aligned overwrite mapping) then the filesystem will use any available
hardware support to avoid the filesystem metadata updates.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>

0 files changed, 0 insertions, 0 deletions