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

Ira Weiny
2019-06-12T22:12:18Z

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.

Ira