Re: Better interop for NFS/SMB file share mode/reservation

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Mar 7, 2019 at 1:04 PM Stefan Metzmacher <metze@samba.org> 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.

Getting back to this.
Thanks all for the valuable inputs.

Next week is LSF/MM and I was assigned a 30 minute slot on filesystems track
to discuss "NFS/SMB file share".

So let me try to echo what I read on this thread and how I understand what APIs
samba needs from the kernel.

>
> If we want to design an useful API, we also need to think about
> all features:
> - file oplock/leases

Kernel can have a flavor of leases which are not broken
by opens from threads of the process holding the lease.
Bruce has some patches along those lines for knfsd and SMB R/RW
leases could use this flavor if it was exported to userspace?

For SMB RH/RWH leases and Ganesha delegations, server
could keep track of its own handles/clients and break leases within the
same process without involving the kernel.
Am I wrong?

> - directory leases

I have WIP on fsnotify directory pre modification hooks.
There is opposition from fsnotify maintainer to add new userspace
APIs that can create kernel->user->kernel deadlocks, like the
deadlocks currently reported with fanotify permission events.

Need to see if we can find a middle ground between
"post modification notifications" and "pre modification permission"
API, somewhere along the lines of regular file lease breaking API.

> - share modes

Volker told me he thinks samba can enforce share modes by
a single daemon policing all opens in the system with fanotify.
I think he is right. If anyone thinks differently please speak up.

> - 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

So this interface exists in the kernel.
Nothing more required from the kernel API. Right?

> - 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.

I should hope that the new AIO API (http://kernel.dk/io_uring.pdf)
would solve those problems as well as other issues that
samba has w.r.t dispatching AIO.

>
> 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.
>

Tested interfaces - good idea ;-)

If anyone has any comments about my view of required new interfaces,
or important things that I missed, please say so before Tuesday!

Thanks,
Amir.