Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory

[Jason Gunthorpe]

On Thu, Feb 23, 2023 at 09:18:23AM -0800, T.J. Mercier wrote:

> > Solving that problem means figuring out when every cgroup stops using
> > the memory - pinning or not. That seems to be very costly.
> >
> This is the current behavior of accounting for memfds, and I suspect
> any kind of shared memory.
> 
> If cgroup A creates a memfd, maps and faults in pages, shares the
> memfd with cgroup B and then A unmaps and closes the memfd, then
> cgroup A is still charged for the pages it faulted in.

As we discussed, as long as the memory is swappable then eventually
memory pressure on cgroup A will evict the memfd pages and then cgroup
B will swap it in and be charged for it.
 
> FWIW this is also the behavior I was trying to use to attribute
> dma-buffers to their original allocators. Whoever touches it first
> gets charged as long as the memory is alive somewhere.
>
> Can't we do the same thing for pins?

If pins are tracked independently from memcg then definately not,
a process in cgroup A should never be able to make a charge on cgroup
B as a matter of security.

If pins are part of the memcg then we can't always turn the pin
request in to a NOP - the current cgroup always has to be charged for
the memory. Otherwise what is the point from a security perspective?

Jason