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

[Yosry Ahmed]

On Thu, Feb 23, 2023 at 9:28 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> 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.

I am not familiar with memfd, but based on
mem_cgroup_swapin_charge_folio() it seems like if cgroup B swapped in
the pages they will remain charged to cgroup A, unless cgroup A is
removed/offlined. Am I missing something?

>
> > 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