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

[Jan Kara]

On Wed 12-06-19 15:13:36, Ira Weiny 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.

Yes, I repeat we are in a situation when the application has a bug and
didn't propely manage its long term pins which are fully under its control.
So in my mind a situation similar to application using memory it has
already freed. The kernel has to manage that but we don't really care
what's left from the application when this happens.

That being said I'm not insisting this has to happen - tracking associated
"RDMA file" with a layout lease and somehow invalidating it on close of a
leased file is somewhat ugly anyway. But it is still an option if exposing
pins to userspace for lsof to consume proves even worse...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR