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Date: Mon, 14 Apr 2014 08:43:16 +0200
From: Alexander Graf <agraf@suse.de>
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To: Liu ping fan <kernelfans@gmail.com>
Subject: Re: [PATCH v3] powerpc: kvm: make _PAGE_NUMA take effect
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Cc: Liu Ping Fan <pingfank@linux.vnet.ibm.com>,
 "kvm@vger.kernel.org mailing list" <kvm@vger.kernel.org>,
 kvm-ppc <kvm-ppc@vger.kernel.org>, Paul Mackerras <paulus@samba.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
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On 13.04.14 04:27, Liu ping fan wrote:
> On Fri, Apr 11, 2014 at 10:03 PM, Alexander Graf <agraf@suse.de> wrote:
>> On 11.04.2014, at 13:45, Liu Ping Fan <pingfank@linux.vnet.ibm.com> wrote:
>>
>>> When we mark pte with _PAGE_NUMA we already call mmu_notifier_invalidate_range_start
>>> and mmu_notifier_invalidate_range_end, which will mark existing guest hpte
>>> entry as HPTE_V_ABSENT. Now we need to do that when we are inserting new
>>> guest hpte entries.
>> What happens when we don't? Why do we need the check? Why isn't it done implicitly? What happens when we treat a NUMA marked page as non-present? Why does it work out for us?
>>
>> Assume you have no idea what PAGE_NUMA is, but try to figure out what this patch does and whether you need to cherry-pick it into your downstream kernel. The description as is still is not very helpful for that. It doesn't even explain what really changes with this patch applied.
>>
> Yeah.  what about appending the following description?  Can it make
> the context clear?
> "Guest should not setup a hpte for the page whose pte is marked with
> _PAGE_NUMA, so on the host, the numa-fault mechanism can take effect
> to check whether the page is placed correctly or not."

Try to come up with a text that answers the following questions in order:

   - What does _PAGE_NUMA mean?
   - How does page migration with _PAGE_NUMA work?
   -> Why should we not map pages when _PAGE_NUMA is set?
   - Which part of what needs to be done did the previous _PAGE_NUMA 
patch address?
   - What's the situation without this patch?
   - Which scenario does this patch fix?

Once you have a text that answers those, you should have a good patch 
description :).

Alex