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@xxxxxxxxxx> Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx> Reviewed-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx> --- Documentation/filesystems/fscrypt.rst | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst index 44b67ebd6e40..c0c1747fa2fb 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 starting position in the file and 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 ============================= -- 2.32.0.rc1.229.g3e70b5a671-goog
