Better interop for NFS/SMB file share mode/reservation

[Amir Goldstein]

On Sat, Feb 9, 2019 at 12:12 AM Jeremy Allison <jra at samba.org> wrote:
>
> On Fri, Feb 08, 2019 at 10:02:43PM +0200, Amir Goldstein via samba-technical wrote:
> > On Fri, Feb 8, 2019 at 5:51 PM J. Bruce Fields <bfields at fieldses.org> wrote:
> > >
> > > On Fri, Feb 08, 2019 at 04:45:46PM +0200, Amir Goldstein wrote:
> > > > - check_conflicting_open() is changed to use inode_is_open_for_read()
> > > >   instead of checking d_count and i_count.
> > >
> > > Independently of the rest, I'd love to do away with those
> > > d_count/i_count checks.  What's inode_is_open_for_read()?
> > >
> >
> > It would look maybe something like this:
> >
> > static inline bool file_is_open_for_read(const struct inode *file)
> > {
> >         struct inode *inode = file_inode(file);
> >         int countself = (file->f_mode & (FMODE_READ | FMODE_WRITE)) ==
> > FMODE_READ) ? 1 : 0;
> >
> >         return atomic_read(&inode->i_readcount) > countself;
> > }
> >
> > And it would allow for acquiring F_WRLCK lease if other
> > instances of inode are open O_PATH.
> > A slight change of semantics that seems harmless(?)
> > and will allow some flexibility.
> >
> > But if samba can't figure out a way to keep a single open file
> > descriptor for oplocks per client-file, then this model doesn't
> > help us make any progress.
>
> Samba uses a single file descriptor per SMB2 open file
> handle. Is this what you meant ? We need this to keep
> the per-handle OFD locks around.

I understand now there are several cases when smbd has
several open file descriptors for the same client.
Is that related to this comment in samba wiki about kernel oplocks?

"Linux kernel oplocks don't provide the needed features.
(They don't even work correctly for oplocks...)"

Can you elaborate on that? is that because a samba oplock
is per client and therefore other file opens from same client
should not break its own lease?
If that is the case, than Bruce's work on the "delegations"
flavor of kernel oplocks could make them a good fit for samba.

As Bruce wrote, we could export the "delegations" flavor
to user space for samba, just after we figure out it matches
samba requirements.

Thanks,
Amir.

[1] https://wiki.samba.org/index.php/Samba3/SMB2#locking.2Fopen_files_.28fs_layer.29
[2] https://lore.kernel.org/lkml/1549656647-25115-1-git-send-email-bfields@redhat.com/