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[v2,7/7] fscrypt: update documentation for direct I/O support

Message ID 20200717013518.59219-8-satyat@google.com
State Superseded
Headers show
Series add support for direct I/O with fscrypt using blk-crypto | expand

Commit Message

Satya Tangirala July 17, 2020, 1:35 a.m. UTC
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(-)
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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.