Better interop for NFS/SMB file share mode/reservation

[Simo]

On Thu, 2019-03-07 at 12:03 +0100, Stefan Metzmacher via samba-
technical wrote:
> Am 06.03.19 um 22:25 schrieb Ralph Böhme via samba-technical:
> > Jeremy Allison wrote:
> > > On Wed, Mar 06, 2019 at 03:31:08PM -0500, Jeff Layton wrote:
> > > > On Wed, 2019-03-06 at 10:11 -0500, J. Bruce Fields wrote:
> > > > > Jeff, wasn't there some work (on Ceph maybe?) on a userspace delegation
> > > > > API?  Is that close to what's needed?
> > > > > 
> > > > 
> > > > Here's the C headers for that stuff:
> > > > 
> > > >    https://github.com/ceph/ceph/blob/7ba6bece4187eda5d05a9b84211fe6ba8dd287bd/src/include/cephfs/libcephfs.h#L1734
> > > > 
> > > > It's simple enough and works for us in ganesha, and I think we can
> > > > probably adapt it to samba without too much difficulty. The callback
> > > > doesn't seem like it'll do for a kernel API though -- you'd almost
> > > > certainly need to do something different there (signals? inotify?).
> > > 
> > > SMB3 leases have R/RW and Handle-based leases.
> > 
> > Just to be precise: SMB2.1+ has R, RH, RW and RWH leases.
> > 
> > > Handle leases allow multiple opens of the same pathname
> > > that get different handles to share the lease, allowing
> > > a client redirector to delay opens or closes locally
> > > so long as it has a handle lease.
> > 
> > That'a a propertly of leases in general, not just H-leases. The client provides a lease key which is a GUID with each lease request
> > 
> > > Here are the semantics:
> > > 
> > > https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-smb2/d8df943d-6ad7-4b30-9f58-96ae90fc6204
> > > 
> > > I'm not sure a simple file-descriptor based API is
> > > enough for us. Can he have a uuid or token based
> > > API instead where the server can chose what fd's
> > > to cover with a token ?
> > 
> > Yes, that would be ideal.
> 
> If we want to design an useful API, we also need to think about
> all features:
> - file oplock/leases
> - directory leases
> - share modes
> - disconnected handles (for durable and persistent handles),
>   which exists within the kernel for a while and can be reattached
>   to process, using some kind of cookie and the same euid
> - the API needs ways to use epoll in order to do async opens
>   and lease breaks. For opens the model of async socket connects
>   could be used. Leases could have a signalfd-style api.
> 
> We may not need everything at once, but we should have the full picture
> in mind. And we need working code in kernel and userspace that passes
> all tests (we may need to add additional test). Otherwise the kernel
> creates new syscalls, which wouldn't be used by Samba in the end.

Just a thought, but you should probably classify these facilities in
two lists, one for items that can only reasonably be done via a kernel
API and one for items that can be satisfactorily be handled via a
coordinating userspace component (daemon/database/convention/other).

Getting all that stuff in kernel may prove overly hard and contentious
so being able to negotiate on the critical items only may be important.

Simo.