From: Jaegeuk Kim <jaegeuk.kim@samsung.com>
To: "'Dave Chinner'" <david@fromorbit.com>,
        "'Jaegeuk Kim'" <jaegeuk.kim@gmail.com>
Cc: "'Arnd Bergmann'" <arnd@arndb.de>,
        "'Vyacheslav Dubeyko'" <slava@dubeyko.com>, viro@zeniv.linux.org.uk,
        "'Theodore Ts'o'" <tytso@mit.edu>, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, chur.lee@samsung.com,
        cm224.lee@samsung.com, jooyoung.hwang@samsung.com
References: <001201cda2f1$633db960$29b92c20$%kim@samsung.com>
 <20121015223409.GE2739@dastard>
 <015901cdab42$06deac20$149c0460$%kim@samsung.com>
 <201210161138.35388.arnd@arndb.de> <20121016204412.GF2864@dastard>
 <1350426621.1958.119.camel@kjgkr> <20121016225410.GA26797@dastard>
In-reply-to: <20121016225410.GA26797@dastard>
Subject: RE: [PATCH 11/16] f2fs: add inode operations for special inodes
Date: Wed, 17 Oct 2012 12:25:46 +0900
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> On Wed, Oct 17, 2012 at 07:30:21AM +0900, Jaegeuk Kim wrote:
> > > > > OTOH, I think xattr itself is for users, not for communicating
> > > > > between file system and users.
> > > >
> > > > No, you are mistaken in that point, as Dave explained.
> > >
> > > e.g. selinux, IMA, ACLs, capabilities, etc all communicate
> > > information that the kernel uses for access control. That's why
> > > xattrs have different namespaces like "system", "security" and
> > > "user". Only user attributes are truly for user data
> > > - the rest are for communicating information to the kernel....
> > >
> >
> > I agree that "system" is used by kernel.
> > How about the file system view?
> 
> Not sure what you mean - the filesystem woul dsimply read the xattrs
> in the system namespace as it needs, just like the other subsystems
> like selinux or IMA do.
> 
> > Would you explain what file systems retrieve xattrs and use
> > them with their own purpose?
> 
> I think cachefs users a "CacheFiles.cache" namespace for storing
> information it needs in xattrs. ecryptfs stores crypto metadata in
> xattrs in the lower filesytem. NFSv4 servers store junction mount
> information in xattrs.
> 
> So there are examples where filesystems use xattrs for special
> information. However, in most cases filesystems don't need xattrs
> for their own metadata primarily because that gets added to their
> own on-disk formats. IThe above are all "overlay" style filesystems
> that don't have their own on-disk formats, so need to use xattrs to
> store their per-inode metadata.
> 
> The case of access hints and allocation policies are not somethign
> that are native to any filesystem on-disk format. They are abstract
> concepts that really only the software generating/using that
> information knows about. Given we want the software that uses this
> information to be in VFS, it is separate from every filesystem and
> this is exactly the use case that system xattrs were intended for.
> :)

I understand. Thank you very much. :)

> 
> > Sorry, I'm not familiar with xattrs in depth.
> >
> > Unfortunately, "system" is not implemented in f2fs yet. :(
> 
> If you've already implemented the user.* namespace, then it's
> trivial to support the other namespaces - it's just prefixing the
> xattrs with the appropriate string instead of "user"....
> 

Ok, I'll do right now.
Thanks, again.

> Cheers,
> 
> Dave.
> --
> Dave Chinner
> david@fromorbit.com

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