Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     Michal Hocko <mhocko@kernel.org>, virtio-dev@lists.oasis-open.org,
        kvm list <kvm@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>, will@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Oscar Salvador <osalvador@suse.de>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Nitesh Narayan Lal <nitesh@redhat.com>,
        Rik van Riel <riel@surriel.com>, lcapitulino@redhat.com,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>, ying.huang@intel.com,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
References: <20190907172225.10910.34302.stgit@localhost.localdomain>
 <20190910124209.GY2063@dhcp22.suse.cz>
 <CAKgT0Udr6nYQFTRzxLbXk41SiJ-pcT_bmN1j1YR4deCwdTOaUQ@mail.gmail.com>
 <20190910144713.GF2063@dhcp22.suse.cz>
 <CAKgT0UdB4qp3vFGrYEs=FwSXKpBEQ7zo7DV55nJRO2C-KCEOrw@mail.gmail.com>
 <20190910161818.GF2797@work-vm>
From:   David Hildenbrand <david@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI
 BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG
 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj
 xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3
 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob
 s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM
 m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez
 MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK
 z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh
 dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E
 UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq
 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga
 uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid
 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X
 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+
 xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC
 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb
 hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC
 u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS
 gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg
 rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl
 BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl
 KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ
 NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo
 YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5
 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy
 qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl
 C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV
 W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV
 TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx
 +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl
 SE+xAvmumFBY
Organization: Red Hat GmbH
Message-ID: <f74117db-225d-92cb-9476-22c0f752659d@redhat.com>
Date:   Tue, 10 Sep 2019 18:22:37 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20190910161818.GF2797@work-vm>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10.09.19 18:18, Dr. David Alan Gilbert wrote:
> * Alexander Duyck (alexander.duyck@gmail.com) wrote:
>> On Tue, Sep 10, 2019 at 7:47 AM Michal Hocko <mhocko@kernel.org> wrote:
>>>
>>> On Tue 10-09-19 07:42:43, Alexander Duyck wrote:
>>>> On Tue, Sep 10, 2019 at 5:42 AM Michal Hocko <mhocko@kernel.org> wrote:
>>>>>
>>>>> I wanted to review "mm: Introduce Reported pages" just realize that I
>>>>> have no clue on what is going on so returned to the cover and it didn't
>>>>> really help much. I am completely unfamiliar with virtio so please bear
>>>>> with me.
>>>>>
>>>>> On Sat 07-09-19 10:25:03, Alexander Duyck wrote:
>>>>> [...]
>>>>>> This series provides an asynchronous means of reporting to a hypervisor
>>>>>> that a guest page is no longer in use and can have the data associated
>>>>>> with it dropped. To do this I have implemented functionality that allows
>>>>>> for what I am referring to as unused page reporting
>>>>>>
>>>>>> The functionality for this is fairly simple. When enabled it will allocate
>>>>>> statistics to track the number of reported pages in a given free area.
>>>>>> When the number of free pages exceeds this value plus a high water value,
>>>>>> currently 32, it will begin performing page reporting which consists of
>>>>>> pulling pages off of free list and placing them into a scatter list. The
>>>>>> scatterlist is then given to the page reporting device and it will perform
>>>>>> the required action to make the pages "reported", in the case of
>>>>>> virtio-balloon this results in the pages being madvised as MADV_DONTNEED
>>>>>> and as such they are forced out of the guest. After this they are placed
>>>>>> back on the free list,
>>>>>
>>>>> And here I am reallly lost because "forced out of the guest" makes me
>>>>> feel that those pages are no longer usable by the guest. So how come you
>>>>> can add them back to the free list. I suspect understanding this part
>>>>> will allow me to understand why we have to mark those pages and prevent
>>>>> merging.
>>>>
>>>> Basically as the paragraph above mentions "forced out of the guest"
>>>> really is just the hypervisor calling MADV_DONTNEED on the page in
>>>> question. So the behavior is the same as any userspace application
>>>> that calls MADV_DONTNEED where the contents are no longer accessible
>>>> from userspace and attempting to access them will result in a fault
>>>> and the page being populated with a zero fill on-demand page, or a
>>>> copy of the file contents if the memory is file backed.
>>>
>>> As I've said I have no idea about virt so this doesn't really tell me
>>> much. Does that mean that if somebody allocates such a page and tries to
>>> access it then virt will handle a fault and bring it back?
>>
>> Actually I am probably describing too much as the MADV_DONTNEED is the
>> hypervisor behavior in response to the virtio-balloon notification. A
>> more thorough explanation of it can be found by just running "man
>> madvise", probably best just to leave it at that since I am probably
>> confusing things by describing hypervisor behavior in a kernel patch
>> set.
>>
>> For the most part all the page reporting really does is provide a way
>> to incrementally identify unused regions of memory in the buddy
>> allocator. That in turn is used by virtio-balloon in a polling thread
>> to report to the hypervisor what pages are not in use so that it can
>> make a decision on what to do with the pages now that it knows they
>> are unused.
>>
>> All this is providing is just a report and it is optional if the
>> hypervisor will act on it or not. If the hypervisor takes some sort of
>> action on the page, then the expectation is that the hypervisor will
>> use some sort of mechanism such as a page fault to discover when the
>> page is used again.
> 
> OK, that's interestingly different (but OK) from some other schemes that
> hav ebeen described which *require* the guest to somehow indicate the
> page is in use before starting to use the page again.
> 

virtio-balloon also has a mode where the guest would not have to
indicate to the host before re-using a page. Only
VIRTIO_BALLOON_F_MUST_TELL_HOST enforces this. So it's not completely new.

> Dave


-- 

Thanks,

David / dhildenb