From: Peter Staubach <staubach@redhat.com>
Subject: Re: [PATCH] 64 bit ino support for NFS server
Date: Wed, 08 Aug 2007 16:49:00 -0400
Message-ID: <46BA2C3C.6040504@redhat.com>
References: <46B37DE6.80706@redhat.com> <46B38206.6050504@redhat.com>
	<20070804223256.GA1155@fieldses.org> <46BA2289.5060902@redhat.com>
	<20070808203555.GL16171@fieldses.org>
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Cc: Neil Brown <neilb@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	NFS List <nfs@lists.sourceforge.net>
To: "J. Bruce Fields" <bfields@fieldses.org>
In-Reply-To: <20070808203555.GL16171@fieldses.org>
Sender: nfs-bounces@lists.sourceforge.net
Errors-To: nfs-bounces@lists.sourceforge.net

J. Bruce Fields wrote:
> On Wed, Aug 08, 2007 at 04:07:37PM -0400, Peter Staubach wrote:
>   
>> I'm testing a new version of the patch, which addresses this
>> issue, but then I got to wondering, what is lease_get_mtime()
>> really for and is it solving a real problem, and if so, is it
>> solving it in a reasonable fashion?
>>
>> It appears to me that it is used to detect when an application
>> on the server has acquired an exclusive lease for a file and
>> may be modifying it, but without informing the kernel.  If it
>> was informing the kernel, then the mtime would be updated.
>>
>> NFS doesn't support leases like this, so it would need to be
>> an application running on the server, either as a standalone
>> application or as a server for some other service.
>>     
>
> Let's say it's Samba, since that's probably what is is.
>
> So Samba is holding a write lease on the file on behalf of one of its
> clients.  We never see an open from our NFSv2/v3 client--if it doesn't
> have data cached, we may never even see a read, just access and
> getattr--so we don't know to break that lease.
>
> (Actually, if a write lease, like an NFSv4 write delegation, is meant to
> cover file attributes as well as data, then perhaps stat should break
> write leases.  That sounds painful.)
>
> Anyway, the delay between the time the Samba client makes a change and
> the time the NFS client sees it could be unbounded.  Whereas with
> lease_get_mtime() the client will invalidate its cache and perform a
> read, which *does* break the lease (in nfsd_open).
>
> But I agree that this is a strange compromise, and I'd like to
> understand better what the semantics for sharing with a Samba client are
> supposed to be.
>
>   

Yes.

>> And, why does it matter whether the attributes are being
>> returned via GETATTR, via a post_op_attr, or via a post_op_attr
>> as part of a wcc_data?  The first two cases invoke lease_get_mtime(),
>> while the third does not.
>>     
>
> What's the code path in the third case?
>
>   

The support in fill_post_wcc() doesn't invoke lease_get_mtime().

    Thanx...

       ps

>> Could this not lead to time jumping around on a particular file,
>> forwards and backwards?
>>     
>
> Yeah, sounds bad.
>
> --b.
>   


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