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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 28 Mar 2019 20:59:45 -0700
Message-ID: <CALvZod5GiC1+HB3_Mm969Qbgj7s6-unbd141uP5pnMbsufS+mg@mail.gmail.com>
Subject: Re: [RFC PATCH] mm, kvm: account kvm_vcpu_mmap to kmemcg
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ben Gardon <bgardon@google.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Linux MM <linux-mm@kvack.org>, kvm@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Mar 28, 2019 at 7:36 PM Matthew Wilcox <willy@infradead.org> 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