[PATCH v3 7/7] fscrypt: update documentation for direct I/O support

[Satya Tangirala <satyat@google.com>]

Update fscrypt documentation to reflect the addition of direct I/O support
and document the necessary conditions for direct I/O on encrypted files.

Signed-off-by: Satya Tangirala <satyat@google.com>
---
 Documentation/filesystems/fscrypt.rst | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index f3d87a1a0a7f..95c76a5f0567 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -1049,8 +1049,10 @@ astute users may notice some differences in behavior:
   may be used to overwrite the source files but isn't guaranteed to be
   effective on all filesystems and storage devices.
 
-- Direct I/O is not supported on encrypted files.  Attempts to use
-  direct I/O on such files will fall back to buffered I/O.
+- Direct I/O is supported on encrypted files only under some circumstances
+  (see `Direct I/O support`_ for details). When these circumstances are not
+  met, attempts to use direct I/O on such files will fall back to buffered
+  I/O.
 
 - The fallocate operations FALLOC_FL_COLLAPSE_RANGE and
   FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will
@@ -1257,6 +1259,20 @@ without the key is subject to change in the future.  It is only meant
 as a way to temporarily present valid filenames so that commands like
 ``rm -r`` work as expected on encrypted directories.
 
+Direct I/O support
+------------------
+
+Direct I/O on encrypted files is supported through blk-crypto. In
+particular, this means the kernel must have CONFIG_BLK_INLINE_ENCRYPTION
+enabled, the filesystem must have had the 'inlinecrypt' mount option
+specified, and either hardware inline encryption must be present, or
+CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK must have been enabled. Further,
+any I/O must be aligned to the filesystem block size (*not* necessarily
+the same as the block device's block size) - in particular, any userspace
+buffer into which data is read/written from must also be aligned to the
+filesystem block size. If any of these conditions isn't met, attempts to do
+direct I/O on an encrypted file will fall back to buffered I/O.
+
 Tests
 =====
 
-- 
2.28.0.rc0.105.gf9edc3c819-goog