VOP_LEASE

[Jeff Roberson]

On Sat, 12 Apr 2008, Alfred Perlstein wrote:

> * Jeff Roberson <jroberson at jroberson.net> [080412 16:13] wrote:
>> On Sat, 12 Apr 2008, Doug Rabson wrote:
>>
>>>
>>> On 12 Apr 2008, at 18:03, Kirk McKusick wrote:
>>>
>>>>> Date: Sat, 12 Apr 2008 02:13:15 -1000 (HST)
>>>>> From: Jeff Roberson <jroberson at jroberson.net>
>>>>> To: arch at freebsd.org
>>>>> Subject: VOP_LEASE
>>>>>
>>>>> As far as I can tell this has never been used.  Unless someone can show
>>>>> me
>>>>> otherwise I'm going to go ahead and remove it.
>>>>>
>>>>> Thanks,
>>>>> Jeff
>>>>
>>>> VOP_LEASE is used by NQNFS and NFSv4. It notifies them when a file
>>>> is modified locally so that they know to update any outstanding
>>>> leases (e.g., evict any write lease for the file and do callbacks
>>>> for any read leases for the file). Deleting VOP_LEASE would break
>>>> NFS big time.
>>>
>>> I think our NQNFS support might have been removed some time ago - I can't
>>> see any calls to VOP_LEASE in the code right now. Something like VOP_LEASE
>>> would certainly be useful for a hypothetical future NFSv4 server. I
>>> believe that samba could use it too for its oplocks feature which appears
>>> to be similar to NQNFS's leases and NFSv4's delegations.
>>
>> So the idea with delegations is that close() doesn't actually release the
>> file entirely to make future access cheaper?
>>
>> My issue with VOP_LEASE is only that there are no in kernel
>> implementations of the VOP.  I doubt it is applied regularly in syscalls.
>> It also seems odd that it is called without a lock.
>>
>> Is the intent that the server will trap all accesses to a local vnode in
>> order to invalidate the client leases?
>
> VOP_LEASE is supposed to implemented by a filesystem client.
>
> For insance, NFS client with NQNFS would implement the VOP_LEASE
> and trap those accesses to manage the lease with the remote server.
>
> The remote server would get "lease RPCs" from the client and manage
> the cache appropriately.

So why isn't this done within the actual VOP?  If the lease expires 
between calling VOP_LEASE and vn_lock(), VOP_READ() you have to do that 
work all over again anyway.

I don't yet see why this is in filesystem independent code.  I'm not 
asserting that it doesn't need to be.  I'd just like to understand it 
better.

Thanks,
Jeff

>
> -- 
> - Alfred Perlstein
>