linux-kernel - Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

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Message-ID: <20100706232351.GD25018@dastard>
Date:	Wed, 7 Jul 2010 09:23:51 +1000
From:	Dave Chinner <david@...morbit.com>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....com, corbet@....net,
	serue@...ibm.com, hooanon05@...oo.co.jp,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls

On Tue, Jul 06, 2010 at 10:39:56PM +0530, Aneesh Kumar K. V wrote:
> On Tue, 6 Jul 2010 12:10:02 -0400, "J. Bruce Fields" <bfields@...ldses.org> wrote:
> > On Fri, Jul 02, 2010 at 02:45:45AM +0530, Aneesh Kumar K. V wrote:
> > > One use case i had was that if the userspace file server can directly
> > > work with the returned file system UUID,
> > 
> > I agree that the uuid should be split out from the rest of the
> > filehandle, but ...
> > 
> > > the it can build the file
> > > handle for client in a single call.
> > 
> > ... I don't understand why both need to come in the same system call.
> > Is it purely an efficiency question?  If so, why do you expect this to
> > be significant?
> 
> Since we know that system wide file handle should include a file system
> identifier and a file identifier my plan was to retrieve both in the
> same syscall.
> 
> 
> > 
> > (I would have thought that the system call overhead is so small, and so
> > many calls will already be required to perform the typical rpc, that
> > this would be insignificant.)
> > 
> > A filesystem uuid seems like a generally useful thing (maybe more so
> > than a filehandle), so it'd seem worth figuring out how to export that
> > separately.
> > 
> 
> I can add a new syscall that returns 
> 
> struct fs_uuid {
>        u8 fs_uuid[16];  
> };
> 
> long sys_get_fs_uuid(int dfd, char *name, struct fs_uuid *fsid, int flag);

libblkid already provides the UUID to userspace applications, doesn't it?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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