Re: [RFC] Make use of non-dynamic dmabuf in RDMA

From: Jason Gunthorpe
Date: Wed Aug 25 2021 - 10:47:43 EST

On Wed, Aug 25, 2021 at 03:51:14PM +0200, Christian König wrote:
> Am 25.08.21 um 14:38 schrieb Jason Gunthorpe:
> > On Wed, Aug 25, 2021 at 02:27:08PM +0200, Christian König wrote:
> > > Am 25.08.21 um 14:18 schrieb Jason Gunthorpe:
> > > > On Wed, Aug 25, 2021 at 08:17:51AM +0200, Christian König wrote:
> > > >
> > > > > The only real option where you could do P2P with buffer pinning are those
> > > > > compute boards where we know that everything is always accessible to
> > > > > everybody and we will never need to migrate anything. But even then you want
> > > > > some mechanism like cgroups to take care of limiting this. Otherwise any
> > > > > runaway process can bring down your whole system.
> > > > Why? It is not the pin that is the problem, it was allocating GPU
> > > > dedicated memory in the first place. pinning it just changes the
> > > > sequence to free it. No different than CPU memory.
> > > Pinning makes the memory un-evictable.
> > >
> > > In other words as long as we don't pin anything we can support as many
> > > processes as we want until we run out of swap space. Swapping sucks badly
> > > because your applications become pretty much unuseable, but you can easily
> > > recover from it by killing some process.
> > >
> > > With pinning on the other hand somebody sooner or later receives an -ENOMEM
> > > or -ENOSPC and there is no guarantee that this goes to the right process.
> > It is not really different - you have the same failure mode once the
> > system runs out of swap.
> >
> > This is really the kernel side trying to push a policy to the user
> > side that the user side doesn't want..
> But which is still the right thing to do as far as I can see. See userspace
> also doesn't want proper process isolation since it takes extra time.

Why? You are pushing a policy of resource allocation/usage which
more properly belongs in userspace.

> Kernel development is driven by exposing the hardware functionality in a
> save and manageable manner to userspace, and not by fulfilling userspace
> requirements.

I don't agree with this, that is a 1980's view of OS design. So much
these days in the kernel is driven entirely by boutique userspace
requirements and is very much not about the classical abstract role of
an OS.

> > Dedicated systems are a significant use case here and should be
> > supported, even if the same solution wouldn't be applicable to someone
> > running a desktop.
> And exactly that approach is not acceptable.

We have knobs and settings all over the place to allow Linux to
support a broad set of use cases from Android to servers, to HPC. So
long as they can co-exist and the various optional modes do not
critically undermine the security of the kernel, it is well in line
with how things have been evolving in the last 15 years.

Here you are talking entirely about policy to control memory
allocation, which is already well trodden ground for CPU memory.

There are now endless boutique ways to deal with this, it is a very
narrow view to say that GPU memory is so special and different that
only one way can be the correct/allowed way.