Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 06, 2019 at 01:32:04PM -0500, Doug Ledford wrote:
> On Wed, 2019-02-06 at 09:52 -0800, Matthew Wilcox wrote:
> > On Wed, Feb 06, 2019 at 10:31:14AM -0700, Jason Gunthorpe wrote:
> > > On Wed, Feb 06, 2019 at 10:50:00AM +0100, Jan Kara wrote:
> > > 
> > > > MM/FS asks for lease to be revoked. The revoke handler agrees with the
> > > > other side on cancelling RDMA or whatever and drops the page pins. 
> > > 
> > > This takes a trip through userspace since the communication protocol
> > > is entirely managed in userspace.
> > > 
> > > Most existing communication protocols don't have a 'cancel operation'.
> > > 
> > > > Now I understand there can be HW / communication failures etc. in
> > > > which case the driver could either block waiting or make sure future
> > > > IO will fail and drop the pins. 
> > > 
> > > We can always rip things away from the userspace.. However..
> > > 
> > > > But under normal conditions there should be a way to revoke the
> > > > access. And if the HW/driver cannot support this, then don't let it
> > > > anywhere near DAX filesystem.
> > > 
> > > I think the general observation is that people who want to do DAX &
> > > RDMA want it to actually work, without data corruption, random process
> > > kills or random communication failures.
> > > 
> > > Really, few users would actually want to run in a system where revoke
> > > can be triggered.
> > > 
> > > So.. how can the FS/MM side provide a guarantee to the user that
> > > revoke won't happen under a certain system design?
> > 
> > Most of the cases we want revoke for are things like truncate().
> > Shouldn't happen with a sane system, but we're trying to avoid users
> > doing awful things like being able to DMA to pages that are now part of
> > a different file.
> 
> Why is the solution revoke then?  Is there something besides truncate
> that we have to worry about?  I ask because EBUSY is not currently
> listed as a return value of truncate, so extending the API to include
> EBUSY to mean "this file has pinned pages that can not be freed" is not
> (or should not be) totally out of the question.
> 
> Admittedly, I'm coming in late to this conversation, but did I miss the
> portion where that alternative was ruled out?

That's my preferred option too, but the preponderance of opinion leans
towards "We can't give people a way to make files un-truncatable".