Re: [f2fs-dev] [PATCH v2 1/2] f2fs: add compress_mode mount option

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Dec 01, 2020 at 01:08:02PM +0900, Daeho Jeong wrote:
> From: Daeho Jeong <daehojeong@google.com>
> 
> We will add a new "compress_mode" mount option to control file
> compression mode. This supports "fs" and "user". In "fs" mode (default),
> f2fs does automatic compression on the compression enabled files.
> In "user" mode, f2fs disables the automaic compression and gives the
> user discretion of choosing the target file and the timing. It means
> the user can do manual compression/decompression on the compression
> enabled files using ioctls.
> 
> Signed-off-by: Daeho Jeong <daehojeong@google.com>
> ---
> v2: changed mount option name and added more explanation of mount option
> ---
>  Documentation/filesystems/f2fs.rst | 35 ++++++++++++++++++++++++++++++
>  fs/f2fs/compress.c                 |  2 +-
>  fs/f2fs/data.c                     |  2 +-
>  fs/f2fs/f2fs.h                     | 30 +++++++++++++++++++++++++
>  fs/f2fs/segment.c                  |  2 +-
>  fs/f2fs/super.c                    | 23 ++++++++++++++++++++
>  6 files changed, 91 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
> index b8ee761c9922..5eb8d63439ec 100644
> --- a/Documentation/filesystems/f2fs.rst
> +++ b/Documentation/filesystems/f2fs.rst
> @@ -260,6 +260,13 @@ compress_extension=%s	 Support adding specified extension, so that f2fs can enab
>  			 For other files, we can still enable compression via ioctl.
>  			 Note that, there is one reserved special extension '*', it
>  			 can be set to enable compression for all files.
> +compress_mode=%s	 Control file compression mode. This supports "fs" and "user"
> +			 modes. In "fs" mode (default), f2fs does automatic compression
> +			 on the compression enabled files. In "user" mode, f2fs disables
> +			 the automaic compression and gives the user discretion of
> +			 choosing the target file and the timing. The user can do manual
> +			 compression/decompression on the compression enabled files using
> +			 ioctls.
>  inlinecrypt		 When possible, encrypt/decrypt the contents of encrypted
>  			 files using the blk-crypto framework rather than
>  			 filesystem-layer encryption. This allows the use of
> @@ -810,6 +817,34 @@ Compress metadata layout::
>  	| data length | data chksum | reserved |      compressed data       |
>  	+-------------+-------------+----------+----------------------------+
>  
> +Compression mode
> +--------------------------
> +
> +f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
> +With this option, f2fs provides a choice to select the way how to compress the
> +compression enabled files (refer to "Compression implementation" section for how to
> +enable compression on a regular inode).
> +
> +1) compress_mode=fs
> +This is the default option. f2fs does automatic compression in the writeback of the
> +compression enabled files.
> +
> +2) compress_mode=user
> +This disables the automaic compression and gives the user discretion of choosing the
> +target file and the timing. The user can do manual compression/decompression on the
> +compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
> +ioctls like the below.
> +
> +To decompress a file,
> +
> +fd = open(filename, O_WRONLY, 0);
> +ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
> +
> +To compress a file,
> +
> +fd = open(filename, O_WRONLY, 0);
> +ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
> +

Why doesn't compress_mode=user just cause regular files to not inherit the
compression flag?  Then users could set or clear the compression flag using
FS_IOC_SETFLAGS, without any need for these new ioctls.

- Eric