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

[Jason Gunthorpe]

On Tue, Feb 21, 2023 at 08:07:13AM -1000, Tejun Heo wrote:
> > AFAIK there are few real use cases to establish a pin on MAP_SHARED
> > mappings outside your cgroup. However, it is possible, the APIs allow
> > it, and for security sandbox purposes we can't allow a process inside
> > a cgroup to triger a charge on a different cgroup. That breaks the
> > sandbox goal.
> 
> It seems broken anyway. Please consider the following scenario:

Yes, this is broken like this already today - memcg doesn't work
entirely perfectly for MAP_SHARED scenarios, IMHO.

> > > for whatever reason is determining the pinning ownership or should the page
> > > ownership be attributed the same way too? If they indeed need to differ,
> > > that probably would need pretty strong justifications.
> > 
> > It is inherent to how pin_user_pages() works. It is an API that
> > establishs pins on existing pages. There is nothing about it that says
> > who the page's memcg owner is.
> > 
> > I don't think we can do anything about this without breaking things.
> 
> That's a discrepancy in an internal interface and we don't wanna codify
> something like that into userspace interface. Semantially, it seems like if
> pin_user_pages() wanna charge pinning to the cgroup associated with an fd
> (or whatever), it should also claim the ownership of the pages
> themselves.

Multiple cgroup can pin the same page, so it is not as simple as just
transfering ownership, we need multi-ownership and to really fix the
memcg limitations with MAP_SHARED without an API impact.

You are right that pinning is really just a special case of
allocation, but there is a reason the memcg was left with weak support
for MAP_SHARED and changing that may be more than just hard but an
infeasible trade off..

At least I don't have a good idea how to even approach building a
reasonable datstructure that can track the number of
charges per-cgroup per page. :\

Jason