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Date:   Thu, 28 Mar 2019 19:35:53 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Shakeel Butt <shakeelb@google.com>
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>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-mm@kvack.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] mm, kvm: account kvm_vcpu_mmap to kmemcg
Message-ID: <20190329023552.GV10344@bombadil.infradead.org>
References: <20190329012836.47013-1-shakeelb@google.com>
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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 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?