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

[Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On Thu, Feb 23, 2023 at 10:03:50AM -0800, Yosry Ahmed wrote:
> On Thu, Feb 23, 2023 at 9:28 AM Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> 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?

Ah, I don't know, Tejun said:

"but it can converge when page usage transfers across cgroups
if needed."

Which I assumed was swap related but I don't know how convergence
works.

Jason