VOP_LEASE

[Doug Rabson]

On 13 Apr 2008, at 00:15, Jeff Roberson 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?

I'm working from memory here (too lazy to checkout an old tree). I  
seem to remember that the way this worked is that when an NQNFS server  
granted a lease to a remote client, it arranged things so that any  
local filesystem access to the leased file would first evict the  
remote leaseholder. While the remote client has a valid lease, it is  
free to agressively cache locally as long as it flushes write to the  
server on eviction. The implementation was quite intrusive on the  
server. I can't quite remember where VOP_LEASE came in and the  
documentation is useless.