linux-kernel - Re: [PATCH v2] kvm, mm: account shadow page tables to kmemcg

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Date:   Fri, 29 Jun 2018 16:30:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Greg Thelen <gthelen@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Feiner <pfeiner@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] kvm, mm: account shadow page tables to kmemcg

On Fri 29-06-18 07:02:24, Shakeel Butt wrote:
> The size of kvm's shadow page tables corresponds to the size of the
> guest virtual machines on the system. Large VMs can spend a significant
> amount of memory as shadow page tables which can not be left as system
> memory overhead. So, account shadow page tables to the kmemcg.
> 
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Greg Thelen <gthelen@...gle.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Peter Feiner <pfeiner@...gle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: stable@...r.kernel.org

I am not familiar wtih kvm to judge but if we are going to account this
memory we will probably want to let oom_badness know how much memory
to account to a specific process. Is this something that we can do?
We will probably need a new MM_KERNEL rss_stat stat for that purpose.

Just to make it clear. I am not opposing to this patch but considering
that shadow page tables might consume a lot of memory it would be good
to know who is responsible for it from the OOM perspective. Something to
solve on top of this.

I would also love to see a note how this memory is bound to the owner
life time in the changelog. That would make the review much more easier.

> ---
> Changelog since v1:
> - replaced (GFP_KERNEL|__GFP_ACCOUNT) with GFP_KERNEL_ACCOUNT
> 
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index d594690d8b95..6b8f11521c41 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -890,7 +890,7 @@ static int mmu_topup_memory_cache_page(struct kvm_mmu_memory_cache *cache,
>  	if (cache->nobjs >= min)
>  		return 0;
>  	while (cache->nobjs < ARRAY_SIZE(cache->objects)) {
> -		page = (void *)__get_free_page(GFP_KERNEL);
> +		page = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
>  		if (!page)
>  			return -ENOMEM;
>  		cache->objects[cache->nobjs++] = page;
> -- 
> 2.18.0.rc2.346.g013aa6912e-goog

-- 
Michal Hocko
SUSE Labs