Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

[Dan Williams]

On Wed, Jun 12, 2019 at 4:32 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
>
> On Wed, Jun 12, 2019 at 03:54:19PM -0700, Dan Williams wrote:
> > On Wed, Jun 12, 2019 at 3:12 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
> > >
> > > On Wed, Jun 12, 2019 at 04:14:21PM -0300, Jason Gunthorpe wrote:
> > > > On Wed, Jun 12, 2019 at 02:09:07PM +0200, Jan Kara wrote:
> > > > > On Wed 12-06-19 08:47:21, Jason Gunthorpe wrote:
> > > > > > On Wed, Jun 12, 2019 at 12:29:17PM +0200, Jan Kara wrote:
> > > > > >
> > > > > > > > > The main objection to the current ODP & DAX solution is that very
> > > > > > > > > little HW can actually implement it, having the alternative still
> > > > > > > > > require HW support doesn't seem like progress.
> > > > > > > > >
> > > > > > > > > I think we will eventually start seein some HW be able to do this
> > > > > > > > > invalidation, but it won't be universal, and I'd rather leave it
> > > > > > > > > optional, for recovery from truely catastrophic errors (ie my DAX is
> > > > > > > > > on fire, I need to unplug it).
> > > > > > > >
> > > > > > > > Agreed.  I think software wise there is not much some of the devices can do
> > > > > > > > with such an "invalidate".
> > > > > > >
> > > > > > > So out of curiosity: What does RDMA driver do when userspace just closes
> > > > > > > the file pointing to RDMA object? It has to handle that somehow by aborting
> > > > > > > everything that's going on... And I wanted similar behavior here.
> > > > > >
> > > > > > It aborts *everything* connected to that file descriptor. Destroying
> > > > > > everything avoids creating inconsistencies that destroying a subset
> > > > > > would create.
> > > > > >
> > > > > > What has been talked about for lease break is not destroying anything
> > > > > > but very selectively saying that one memory region linked to the GUP
> > > > > > is no longer functional.
> > > > >
> > > > > OK, so what I had in mind was that if RDMA app doesn't play by the rules
> > > > > and closes the file with existing pins (and thus layout lease) we would
> > > > > force it to abort everything. Yes, it is disruptive but then the app didn't
> > > > > obey the rule that it has to maintain file lease while holding pins. Thus
> > > > > such situation should never happen unless the app is malicious / buggy.
> > > >
> > > > We do have the infrastructure to completely revoke the entire
> > > > *content* of a FD (this is called device disassociate). It is
> > > > basically close without the app doing close. But again it only works
> > > > with some drivers. However, this is more likely something a driver
> > > > could support without a HW change though.
> > > >
> > > > It is quite destructive as it forcibly kills everything RDMA related
> > > > the process(es) are doing, but it is less violent than SIGKILL, and
> > > > there is perhaps a way for the app to recover from this, if it is
> > > > coded for it.
> > >
> > > I don't think many are...  I think most would effectively be "killed" if this
> > > happened to them.
> > >
> > > >
> > > > My preference would be to avoid this scenario, but if it is really
> > > > necessary, we could probably build it with some work.
> > > >
> > > > The only case we use it today is forced HW hot unplug, so it is rarely
> > > > used and only for an 'emergency' like use case.
> > >
> > > I'd really like to avoid this as well.  I think it will be very confusing for
> > > RDMA apps to have their context suddenly be invalid.  I think if we have a way
> > > for admins to ID who is pinning a file the admin can take more appropriate
> > > action on those processes.   Up to and including killing the process.
> >
> > Can RDMA context invalidation, "device disassociate", be inflicted on
> > a process from the outside? Identifying the pid of a pin holder only
> > leaves SIGKILL of the entire process as the remediation for revoking a
> > pin, and I assume admins would use the finer grained invalidation
> > where it was available.
>
> No not in the way you are describing it.  As Jason said you can hotplug the
> device which is "from the outside" but this would affect all users of that
> device.
>
> Effectively, we would need a way for an admin to close a specific file
> descriptor (or set of fds) which point to that file.  AFAIK there is no way to
> do that at all, is there?

You can certainly give the lease holder the option to close the file
voluntarily via the siginfo_t that can be attached to a lease break
signal. But it's not really "close" you want as much as a finer
grained disassociate.

All that said you could require the lease taker opt-in to SIGKILL via
F_SETSIG before marking the lease "exclusive". That effectively
precludes failing truncate, but it's something we can enforce today
and work on finer grained / less drastic escalations over time for
something that should "never" happen.