[v7,8/8] fscrypt: update documentation for direct I/O support

Commit Message

Satya Tangirala Nov. 17, 2020, 2:07 p.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 <[e-mail]>
Reviewed-by: Eric Biggers <[e-mail]>
Reviewed-by: Jaegeuk Kim <[e-mail]>
---
 Documentation/filesystems/fscrypt.rst | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

Comments

Eric Biggers Nov. 18, 2020, 2:38 a.m. UTC | #1

On Tue, Nov 17, 2020 at 02:07:08PM +0000, Satya Tangirala wrote:
> +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,
> +the length of any I/O must be aligned to the filesystem block size
> +(*not* necessarily the same as the block device's 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. However, there aren't any additional
> +requirements on user buffer alignment (apart from those already present
> +when using direct I/O on unencrypted files).

Actually the position in the file the I/O is targeting must be fs-block aligned
too, not just the length of the I/O.

It's only the pointer to the user data buffer that no longer needs to be
fs-block aligned (this changed between v6 and v7).

- Eric

Patch

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 44b67ebd6e40..757b8aa2af9b 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -1047,8 +1047,10 @@  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
@@ -1121,6 +1123,21 @@  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,
+the length of any I/O must be aligned to the filesystem block size
+(*not* necessarily the same as the block device's 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. However, there aren't any additional
+requirements on user buffer alignment (apart from those already present
+when using direct I/O on unencrypted files).
+
 Encryption policy enforcement
 =============================