linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Mon, 18 Feb 2019 10:39:16 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        'Nitesh Narayan Lal' <nitesh@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lcapitulino@...hat.com" <lcapitulino@...hat.com>,
        "pagupta@...hat.com" <pagupta@...hat.com>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "riel@...riel.com" <riel@...riel.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "dodgen@...gle.com" <dodgen@...gle.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "dhildenb@...hat.com" <dhildenb@...hat.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On 02/18/2019 10:36 AM, Wei Wang wrote:
> On 02/15/2019 05:41 PM, David Hildenbrand wrote:
>> On 15.02.19 10:05, Wang, Wei W wrote:
>>> On Thursday, February 14, 2019 5:43 PM, David Hildenbrand wrote:
>>>> Yes indeed, that is the important bit. They must not be put pack to 
>>>> the
>>>> buddy before they have been processed by the hypervisor. But as the 
>>>> pages
>>>> are not in the buddy, no one allocating a page will stumble over 
>>>> such a page
>>>> and try to allocate it. Threads trying to allocate memory will 
>>>> simply pick
>>>> another buddy page instead of "busy waiting" for that page to be 
>>>> finished
>>>> reporting.
>>> What if a guest thread try to allocate some pages but the buddy 
>>> cannot satisfy
>>> because all the pages are isolated? Would it be the same case that 
>>> the guest thread
>>> gets blocked by waiting all the isolated pages to get madvised by 
>>> the host and
>>> returned to the guest buddy, or even worse, some guest threads get 
>>> killed due to oom?
>> Your question targets low memory situations in the guest. I think Nitesh
>> already answered parts of that question somewhere and I'll let him
>> answer it in detail, only a short comment from my side :)
>>
>> I can imagine techniques where the OOM killer can be avoided, but the
>> OOM handler will eventually kick in and handle it.
>>
>> In general your question is valid and we will have to think about a way
>> to avoid that from happening. However, in contrast to your approach
>> blocking on potentially every page that is being hinted, in Nitesh's
>> approach this would only happen when the guest is really low on memory.
>> And the question is in general, if a guest wants to hint if low on
>> memory ("safety buffer").
>
> I think we should forget that the guest is low on memory because
%s/should/shouldn't
> this approach takes all the pages off the list, not because the guest 
> really
> uses up the free memory.
>
> Guest allocating one page could also potentially be blocked until all 
> the pages
> (as opposed to one page) being madvised and returned to the guest buddy.
>
> Best,
> Wei