linux-kernel - Re: [PATCH 17/21] KVM: arm64: Use common code's approach for __GFP

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Date:   Thu, 11 Jun 2020 08:43:59 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Feiner <pfeiner@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Christoffer Dall <christoffer.dall@....com>
Subject: Re: [PATCH 17/21] KVM: arm64: Use common code's approach for
 __GFP_ZERO with memory caches

On Thu, Jun 11, 2020 at 08:59:05AM +0100, Marc Zyngier wrote:
> >diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >index 9398b66f8a87..688213ef34f0 100644
> >--- a/arch/arm64/kvm/mmu.c
> >+++ b/arch/arm64/kvm/mmu.c
> >@@ -131,7 +131,8 @@ static int mmu_topup_memory_cache(struct
> >kvm_mmu_memory_cache *cache, int min)
> > 	if (cache->nobjs >= min)
> > 		return 0;
> > 	while (cache->nobjs < ARRAY_SIZE(cache->objects)) {
> >-		page = (void *)__get_free_page(GFP_PGTABLE_USER);
> >+		page = (void *)__get_free_page(GFP_KERNEL_ACCOUNT |
> 
> This is definitely a change in the way we account for guest
> page tables allocation, although I find it bizarre that not
> all architectures account for it the same way.

It's not intended to be a functional change, i.e. the allocations should
still be accounted:

  #define GFP_PGTABLE_USER  (GFP_PGTABLE_KERNEL | __GFP_ACCOUNT)
  |
  -> #define GFP_PGTABLE_KERNEL        (GFP_KERNEL | __GFP_ZERO)

  == GFP_KERNEL | __GFP_ACCOUNT | __GFP_ZERO

versus 

  #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)

    with __GFP_ZERO explicitly OR'd in

  == GFP_KERNEL | __GFP_ACCOUNT | __GFP_ZERO

I can put the above in the changelog, unless of course it's wrong and I've
missed something.

> It seems logical to me that nested page tables would be accounted
> against userspace, but I'm willing to be educated on the matter.
> 
> Another possibility is that depending on the context, some allocations
> should be accounted on either the kernel or userspace (NV on arm64
> could definitely do something like that). If that was the case,
> maybe moving most of the GFP_* flags into the per-cache flags,
> and have the renaming that Ben suggested earlier.
> 
> Thanks,
> 
>         M.
> -- 
> Jazz is not dead. It just smells funny...