[PATCH v5 7/7] fscrypt: update documentation for direct I/O support

Satya Tangirala
2020-07-24T12:12:18Z

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>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
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 ec81598477fc..5367c03b17bb 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 encrypted
+ 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
@@ -1123,6 +1125,20 @@ It is not currently possible to backup and restore encrypted files
without the encryption key. This would require special APIs which
have not yet been implemented.

+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.
+
Encryption policy enforcement
=============================

--
2.28.0.rc0.142.g3c755180ce-goog