Message-ID: <f74117db-225d-92cb-9476-22c0f752659d@redhat.com>
Date:   Tue, 10 Sep 2019 18:22:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     Michal Hocko <mhocko@...nel.org>, virtio-dev@...ts.oasis-open.org,
        kvm list <kvm@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oscar Salvador <osalvador@...e.de>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Rik van Riel <riel@...riel.com>, lcapitulino@...hat.com,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \

On 10.09.19 18:18, Dr. David Alan Gilbert wrote:
> * Alexander Duyck (alexander.duyck@...il.com) wrote:
>> On Tue, Sep 10, 2019 at 7:47 AM Michal Hocko <mhocko@...nel.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@...nel.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

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