fs: iomap: Atomic write support

Support direct I/O atomic writes by producing a single bio with REQ_ATOMIC
flag set.

Initially FSes (XFS) should only support writing a single FS block
atomically.

As with any atomic write, we should produce a single bio which covers the
complete write length.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
[djwong: clarify a couple of things in the docs]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index 8e6c721d2330..ee790f843cfa 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -513,6 +513,21 @@ IOMAP_WRITE`` with any combination of the following enhancements:
    if the mapping is unwritten and the filesystem cannot handle zeroing
    the unaligned regions without exposing stale contents.
 
+ * ``IOMAP_ATOMIC``: This write is being issued with torn-write
+   protection.
+   Only a single bio can be created for the write, and the write must
+   not be split into multiple I/O requests, i.e. flag REQ_ATOMIC must be
+   set.
+   The file range to write must be aligned to satisfy the requirements
+   of both the filesystem and the underlying block device's atomic
+   commit capabilities.
+   If filesystem metadata updates are required (e.g. unwritten extent
+   conversion or copy on write), all updates for the entire file range
+   must be committed atomically as well.
+   Only one space mapping is allowed per untorn write.
+   Untorn writes must be aligned to, and must not be longer than, a
+   single file block.
+
 Callers commonly hold ``i_rwsem`` in shared or exclusive mode before
 calling this function.