linux-ext4 - Re: Extended file stat: Splitting file- and fs-specific info?

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Date:	Wed, 09 May 2012 16:12:57 +0200
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	Christoph Hellwig <hch@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Dave Chinner <david@...morbit.com>, bfields@...ldses.org,
	smfrench@...il.com, ben@...adent.org.uk,
	Trond.Myklebust@...app.com, roland@...k.frob.com, jra@...ba.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
	linux-ext4@...r.kernel.org, linux-api@...r.kernel.org,
	libc-alpha@...rceware.org
Subject: Re: Extended file stat: Splitting file- and fs-specific info?

On 05/09/2012 03:51 PM, Andreas Dilger wrote:
> On 2012-05-09, at 6:25 AM, Bernd Schubert wrote:
>> On 05/09/2012 02:05 PM, Christoph Hellwig wrote:
>>> On Wed, May 09, 2012 at 01:55:16PM +0200, Bernd Schubert wrote:
>>>> The basic idea of generation numbers is to check if an inode was
>>>> recycled, so only if the tuple of inode-number and generation-number
>>>> matches we still have the same file. Kernel nfs
>>>
>>> NFS does not and should not look at the inode generation.  Except for a
>>> bit of legacy code for the old pre-Linux 2.4 filehandles it looks at the
>>> opaque file handle returned and only interpreted by the filesystem.  Any
>>> userspace NFS server should do the same.
>>
>> Ok, I didn't look how kernel NFS does it for quite some time already...
>> User space NFS only can do it beginning with 2.6.39 - given that user space also needs to support older kernels and other OSs, which might not have open_by_handle, userspace unfortunately cannot entirely rely on that feature.
>
> But even fewer kernels have sys_statxat() in them (i.e. none), so you can rely on that even less than open_by_handle()...

Well, I didn't say that :) In summary, an application needs to try to 
use the open-by-handle call and if that is not supported, it has to fall 
back to traditional stat and generation-number-ioctl. And as I said 
before, open-by-handle very likely removes the requirement for 
generation numbers in sys_statxat().


Cheers,
Bernd

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