Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

[Dave Chinner]

On Wed, Aug 21, 2019 at 11:02:00AM -0700, Ira Weiny wrote:
> On Tue, Aug 20, 2019 at 08:55:15AM -0300, Jason Gunthorpe wrote:
> > On Tue, Aug 20, 2019 at 11:12:10AM +1000, Dave Chinner wrote:
> > > On Mon, Aug 19, 2019 at 09:38:41AM -0300, Jason Gunthorpe wrote:
> > > > On Mon, Aug 19, 2019 at 07:24:09PM +1000, Dave Chinner wrote:
> > > > 
> > > > > So that leaves just the normal close() syscall exit case, where the
> > > > > application has full control of the order in which resources are
> > > > > released. We've already established that we can block in this
> > > > > context.  Blocking in an interruptible state will allow fatal signal
> > > > > delivery to wake us, and then we fall into the
> > > > > fatal_signal_pending() case if we get a SIGKILL while blocking.
> > > > 
> > > > The major problem with RDMA is that it doesn't always wait on close() for the
> > > > MR holding the page pins to be destoyed. This is done to avoid a
> > > > deadlock of the form:
> > > > 
> > > >    uverbs_destroy_ufile_hw()
> > > >       mutex_lock()
> > > >        [..]
> > > >         mmput()
> > > >          exit_mmap()
> > > >           remove_vma()
> > > >            fput();
> > > >             file_operations->release()
> > > 
> > > I think this is wrong, and I'm pretty sure it's an example of why
> > > the final __fput() call is moved out of line.
> > 
> > Yes, I think so too, all I can say is this *used* to happen, as we
> > have special code avoiding it, which is the code that is messing up
> > Ira's lifetime model.
> > 
> > Ira, you could try unraveling the special locking, that solves your
> > lifetime issues?
> 
> Yes I will try to prove this out...  But I'm still not sure this fully solves
> the problem.
> 
> This only ensures that the process which has the RDMA context (RDMA FD) is safe
> with regard to hanging the close for the "data file FD" (the file which has
> pinned pages) in that _same_ process.  But what about the scenario.
> 
> Process A has the RDMA context FD and data file FD (with lease) open.
> 
> Process A uses SCM_RIGHTS to pass the RDMA context FD to Process B.

Passing the RDMA context dependent on a file layout lease to another
process that doesn't have a file layout lease or a reference to the
original lease should be considered a violation of the layout lease.
Process B does not have an active layout lease, and so by the rules
of layout leases, it is not allowed to pin the layout of the file.

> Process A attempts to exit (hangs because data file FD is pinned).
> 
> Admin kills process A.  kill works because we have allowed for it...
> 
> Process B _still_ has the RDMA context FD open _and_ therefore still holds the
> file pins.
> 
> Truncation still fails.
> 
> Admin does not know which process is holding the pin.
> 
> What am I missing?

Application does not hold the correct file layout lease references.
Passing the fd via SCM_RIGHTS to a process without a layout lease
is equivalent to not using layout leases in the first place.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx