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

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 17, 2020 at 01:45:40AM +0000, Satya Tangirala wrote:
> 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.

Nit: "such files" => "encrypted files".

Nit: most of the text in this file is formatted with textwidth=70.

>  
>  - 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.

This is placing "Direct I/O support" as a subsection of the
"Implementation details" section.

But the direct I/O support is more than just an implementation detail.

How about moving it to a top-level section?

I'd probably put it between "Access semantics" and
"Encryption policy enforcement".

- Eric