Re: [RFC PATCH] mm, kvm: account kvm_vcpu_mmap to kmemcg

[Shakeel Butt]

On Thu, Mar 28, 2019 at 7:36 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 28, 2019 at 06:28:36PM -0700, Shakeel Butt wrote:
> > A VCPU of a VM can allocate upto three pages which can be mmap'ed by the
> > user space application. At the moment this memory is not charged. On a
> > large machine running large number of VMs (or small number of VMs having
> > large number of VCPUs), this unaccounted memory can be very significant.
> > So, this memory should be charged to a kmemcg. However that is not
> > possible as these pages are mmapped to the userspace and PageKmemcg()
> > was designed with the assumption that such pages will never be mmapped
> > to the userspace.
> >
> > One way to solve this problem is by introducing an additional memcg
> > charging API similar to mem_cgroup_[un]charge_skmem(). However skmem
> > charging API usage is contained and shared and no new users are
> > expected but the pages which can be mmapped and should be charged to
> > kmemcg can and will increase. So, requiring the usage for such API will
> > increase the maintenance burden. The simplest solution is to remove the
> > assumption of no mmapping PageKmemcg() pages to user space.
>
> The usual response under these circumstances is "No, you can't have a
> page flag bit".
>

I would say for systems having CONFIG_MEMCG_KMEM, a page flag bit is
not that expensive.

> I don't understand why we need a PageKmemcg anyway.  We already
> have an entire pointer in struct page; can we not just check whether
> page->mem_cgroup is NULL or not?

PageKmemcg is for kmem while page->mem_cgroup is used for anon, file
and kmem memory. So, page->mem_cgroup can not be used for NULL check
unless we unify them. Not sure how complicated would that be.

Shakeel