Can someone explain s_xattr field in the superblock?

Can someone explain s_xattr field in the superblock?

[Sam Gandhi]

Hello,

I have seen some file systems like jffs2/ext4 etc setup the s_xattr
field in the superblock structure.
But there are files systems that  do not initialize s_xattr field in
the superblock.

Can someone kindly explain what functionality  does s_xattr on super
block provides? What kinds of things file system wouldn't be able to
do if the superblock doesn't have this initialized?

Thanks.
-Sam

Can someone explain s_xattr field in the superblock?

[Sam Gandhi]

Hello,

[ Sorry if this duplicate to some, I posted this message earlier to
linux-kernel probably was not appropriate ]

I have seen some file systems like jffs2/ext4 etc setup the s_xattr
field in the superblock structure.
But there are files systems that  do not initialize s_xattr field in
the superblock.

Can someone kindly explain what functionality  does s_xattr on super
block provides? What kinds of things file system wouldn't be able to
do if the superblock doesn't have this initialized?

Thanks.

-Sam

Re: Can someone explain s_xattr field in the superblock?

[Jan Kara]

  Hello,

On Tue 31-05-11 11:23:33, Sam Gandhi wrote:
> I have seen some file systems like jffs2/ext4 etc setup the s_xattr
> field in the superblock structure.
> But there are files systems that  do not initialize s_xattr field in
> the superblock.
  Only filesystems that support extended attributes setup s_xattr.

> Can someone kindly explain what functionality  does s_xattr on super
> block provides? What kinds of things file system wouldn't be able to
> do if the superblock doesn't have this initialized?
  It tells VFS what callbacks to call when user tries to obtain/change
extended attributes. If s_xattr is left at NULL, user will get EOPNOTSUPP
or similar error...

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

Re: Can someone explain s_xattr field in the superblock?

[Ted Ts'o]

On Sat, May 28, 2011 at 08:52:41AM -0700, Sam Gandhi wrote:
> 
> I have seen some file systems like jffs2/ext4 etc setup the s_xattr
> field in the superblock structure.
> But there are files systems that  do not initialize s_xattr field in
> the superblock.
> 
> Can someone kindly explain what functionality  does s_xattr on super
> block provides? What kinds of things file system wouldn't be able to
> do if the superblock doesn't have this initialized?

It provides support for extended attributes, which is often
abbreviated as (not surpisingly) xattr.  Some file systems don't
support extended attributes, so they don't need to setup s_xattr.

For more information, read the attr(5) man page: "man 5 attr".

    	 	      	       	       	   - Ted