Re: [RFC 2/8] fs: Reserve inode flag FS_ATOMICWRITES_FL for atomic writes

From: Dave Chinner <david@fromorbit.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	Ojaswin Mujoo <ojaswin@linux.ibm.com>, Jan Kara <jack@suse.cz>,
	Theodore Ts'o <tytso@mit.edu>,
	Matthew Wilcox <willy@infradead.org>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Luis Chamberlain <mcgrof@kernel.org>,
	John Garry <john.g.garry@oracle.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC 2/8] fs: Reserve inode flag FS_ATOMICWRITES_FL for atomic writes
Date: Mon, 4 Mar 2024 11:59:02 +1100	[thread overview]
Message-ID: <ZeUc1ipKMrh+pOn6@dread.disaster.area> (raw)
In-Reply-To: <4c687c1c5322b4eaf0bb173f0b5d58b38fdaa847.1709361537.git.ritesh.list@gmail.com>

On Sat, Mar 02, 2024 at 01:11:59PM +0530, Ritesh Harjani (IBM) wrote:
> This reserves FS_ATOMICWRITES_FL for flags and adds support in
> fileattr to support atomic writes flag & xflag needed for ext4
> and xfs.
> 
> Co-developed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ioctl.c               | 4 ++++
>  include/linux/fileattr.h | 4 ++--
>  include/uapi/linux/fs.h  | 1 +
>  3 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 76cf22ac97d7..e0f7fae4777e 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -481,6 +481,8 @@ void fileattr_fill_xflags(struct fileattr *fa, u32 xflags)
>  		fa->flags |= FS_DAX_FL;
>  	if (fa->fsx_xflags & FS_XFLAG_PROJINHERIT)
>  		fa->flags |= FS_PROJINHERIT_FL;
> +	if (fa->fsx_xflags & FS_XFLAG_ATOMICWRITES)
> +		fa->flags |= FS_ATOMICWRITES_FL;
>  }
>  EXPORT_SYMBOL(fileattr_fill_xflags);
>  
> @@ -511,6 +513,8 @@ void fileattr_fill_flags(struct fileattr *fa, u32 flags)
>  		fa->fsx_xflags |= FS_XFLAG_DAX;
>  	if (fa->flags & FS_PROJINHERIT_FL)
>  		fa->fsx_xflags |= FS_XFLAG_PROJINHERIT;
> +	if (fa->flags & FS_ATOMICWRITES_FL)
> +		fa->fsx_xflags |= FS_XFLAG_ATOMICWRITES;
>  }
>  EXPORT_SYMBOL(fileattr_fill_flags);
>  
> diff --git a/include/linux/fileattr.h b/include/linux/fileattr.h
> index 47c05a9851d0..ae9329afa46b 100644
> --- a/include/linux/fileattr.h
> +++ b/include/linux/fileattr.h
> @@ -7,12 +7,12 @@
>  #define FS_COMMON_FL \
>  	(FS_SYNC_FL | FS_IMMUTABLE_FL | FS_APPEND_FL | \
>  	 FS_NODUMP_FL |	FS_NOATIME_FL | FS_DAX_FL | \
> -	 FS_PROJINHERIT_FL)
> +	 FS_PROJINHERIT_FL | FS_ATOMICWRITES_FL)
>  
>  #define FS_XFLAG_COMMON \
>  	(FS_XFLAG_SYNC | FS_XFLAG_IMMUTABLE | FS_XFLAG_APPEND | \
>  	 FS_XFLAG_NODUMP | FS_XFLAG_NOATIME | FS_XFLAG_DAX | \
> -	 FS_XFLAG_PROJINHERIT)
> +	 FS_XFLAG_PROJINHERIT | FS_XFLAG_ATOMICWRITES)

I'd much prefer that we only use a single user API to set/clear this
flag.

This functionality is going to be tied to using extent size hints on
XFS to indicate preferred atomic IO alignment/size, so applications
are going to have to use the FS_IOC_FS{G,S}ETXATTR APIs regardless
of whether it's added to the FS_IOC_{G,S}ETFLAGS API.

Also, there are relatively few flags left in the SETFLAGS 32-bit
space, so this duplication seems like a waste of the few flags
that are remaining.

-Dave.
-- 
Dave Chinner
david@fromorbit.com