linux-kernel - Re: [RFC PATCH] fs: xattr-based FS_IOC

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Message-ID: <20140107222739.GA22610@thunk.org>
Date:	Tue, 7 Jan 2014 17:27:39 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	Aurelien Jarno <aurelien@...el32.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Rob Browning <rlb@...aultvalue.org>,
	Dave Chinner <david@...morbit.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH] fs: xattr-based FS_IOC_[GS]ETFLAGS interface

On Tue, Jan 07, 2014 at 11:43:56AM -0800, Darrick J. Wong wrote:
> I had thought of indexed inode flags as an alternative to the xattr/string
> parsing thing.  Feature flags make their first appearance as part of a per-FS
> flag-space and are migrated to the common flag-space when there is demand.
> It would also avoid the need for each fs to create its own flag ioctl.
> 
> On the other hand, someone suggested I try remaking IOC_[GS]ETFLAG as an xattr,
> so off I went. :)
> 
> #define FS_IOC_FLAGS_COMMON	0
> #define FS_IOC_FLAGS_COMMON2	1
> #define FS_IOC_FLAGS_EXT4	0xEF53
> 
> struct inode_flag_ioctl {
> 	u32 flag;
> 	u32 value;	/* or u64? */
> };
> #define FS_IOC_GETFLAGS2 _IOR('f', 12, struct inode_flag_ioctl);
> #define FS_IOC_SETFLAGS2 _IOW('f', 13, struct inode_flag_ioctl);

Is having this structure and demultiplexing based on
inode_flag_ioctl.flag really worth it?

I'd just simply introduce two new ioctl's for generic flags:
FS_IOC_COMMON_[GS]ETFLAGS, and then two new ioctl's for each file
system: FS_IOC_EXT4_[GS]ETFLAGS, FS_IOC_BTRFS_[GS]ETFLAGS, etc.

Is this uglier or pretier than using strings?  Eh.... six of one, half
dozen of the other.  I think it's mostly a matter of personal taste.

      	     	       	     	  	 - Ted
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