From: David Hildenbrand <david@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Alexander Duyck <alexander.duyck@gmail.com>,
Michal Hocko <mhocko@kernel.org>,
virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
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>
Subject: Re: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \
Date: Wed, 11 Sep 2019 11:50:53 +0200 [thread overview]
Message-ID: <8923b919-af67-367f-9588-7f0c26a16831@redhat.com> (raw)
In-Reply-To: <20190911051819-mutt-send-email-mst@kernel.org>
On 11.09.19 11:23, Michael S. Tsirkin wrote:
> On Tue, Sep 10, 2019 at 06:22:37PM +0200, David Hildenbrand wrote:
>> 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.
>
> VIRTIO_BALLOON_F_MUST_TELL_HOST is a bit different.
> When it's not set, guest still must tell host about
> pages in use, it just can batch these notifications
> sending them possibly after page has been used.
> So even with VIRTIO_BALLOON_F_MUST_TELL_HOST off you don't
> skip the notification.
>
I don't think so
VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */
commit bf50e69f63d21091e525185c3ae761412be0ba72
Author: Dave Hansen <dave@linux.vnet.ibm.com>
Date: Thu Apr 7 10:43:25 2011 -0700
virtio balloon: kill tell-host-first logic
[...]
But, if the bit is _not_ set, we are under no obligation to
reverse the order; we're under no obligation to do _anything_.
As of now, qemu-kvm defines the bit, but doesn't set it.
Old code simply told the hypervisor afterwards, but only for little
performance gain. It is not strictly necessary.
> From hypervisor point of view, this feature is very much like adding
> page to the balloon and immediately taking it out of the balloon again,
> just doing it in one operation.
>
> The main difference is the contents of the page, which matters
> with poisoning: in that case hypervisor is expected to hand
> back page with the poisoning content. Not so with regular
> deflate where page contents is undefined.
>
> Well and also the new interface is optimized for large chunks
> of memory since we'll likely be dealing with such.
>
>>> Dave
>>
>>
>> --
>>
>> Thanks,
>>
>> David / dhildenb
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2019-09-11 9:51 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-07 17:25 [PATCH v9 0/8] stg mail -e --version=v9 \ Alexander Duyck
2019-09-07 17:25 ` [PATCH v9 1/8] mm: Add per-cpu logic to page shuffling Alexander Duyck
2019-09-09 8:14 ` David Hildenbrand
2019-09-09 15:11 ` Alexander Duyck
2019-09-10 12:11 ` Michal Hocko
2019-09-10 22:14 ` Alexander Duyck
2019-09-10 22:11 ` Alexander Duyck
2019-09-09 9:07 ` Kirill A. Shutemov
2019-09-09 15:12 ` Alexander Duyck
2019-09-07 17:25 ` [PATCH v9 2/8] mm: Adjust shuffle code to allow for future coalescing Alexander Duyck
2019-09-09 8:19 ` David Hildenbrand
2019-09-09 9:47 ` Kirill A. Shutemov
2019-09-09 15:22 ` Alexander Duyck
2019-09-09 15:35 ` Kirill A. Shutemov
2019-09-09 15:37 ` Alexander Duyck
2019-09-09 16:43 ` Alexander Duyck
2019-09-09 17:00 ` Kirill A. Shutemov
2019-09-10 12:20 ` Michal Hocko
2019-09-10 14:48 ` Alexander Duyck
2019-09-07 17:25 ` [PATCH v9 3/8] mm: Move set/get_pcppage_migratetype to mmzone.h Alexander Duyck
2019-09-09 8:22 ` David Hildenbrand
2019-09-09 9:56 ` Kirill A. Shutemov
2019-09-09 18:01 ` Alexander Duyck
2019-09-09 18:12 ` Alexander Duyck
2019-09-10 12:23 ` Michal Hocko
2019-09-10 14:46 ` Alexander Duyck
2019-09-10 17:45 ` Michal Hocko
2019-09-10 20:26 ` Alexander Duyck
2019-09-07 17:25 ` [PATCH v9 4/8] mm: Use zone and order instead of free area in free_list manipulators Alexander Duyck
2019-09-10 12:27 ` Michal Hocko
2019-09-07 17:25 ` [PATCH v9 5/8] arm64: Move hugetlb related definitions out of pgtable.h to page-defs.h Alexander Duyck
2019-09-09 8:52 ` David Hildenbrand
2019-09-09 15:27 ` Alexander Duyck
2019-09-17 17:48 ` Will Deacon
2019-09-17 20:07 ` Alexander Duyck
2019-09-07 17:25 ` [PATCH v9 6/8] mm: Introduce Reported pages Alexander Duyck
2019-09-09 14:42 ` Kirill A. Shutemov
2019-09-09 16:25 ` Alexander Duyck
2019-09-09 16:33 ` Kirill A. Shutemov
2019-09-07 17:26 ` [PATCH v9 7/8] virtio-balloon: Pull page poisoning config out of free page hinting Alexander Duyck
2019-09-09 8:59 ` David Hildenbrand
2019-09-09 15:31 ` Alexander Duyck
2019-09-07 17:26 ` [PATCH v9 8/8] virtio-balloon: Add support for providing unused page reports to host Alexander Duyck
2019-09-07 17:34 ` [PATCH v9 0/8] mm / virtio: Provide support for unused page reporting Alexander Duyck
2019-09-10 12:42 ` [PATCH v9 0/8] stg mail -e --version=v9 \ Michal Hocko
2019-09-10 14:42 ` Alexander Duyck
2019-09-10 14:47 ` Michal Hocko
2019-09-10 16:05 ` Alexander Duyck
2019-09-10 16:18 ` [virtio-dev] " Dr. David Alan Gilbert
2019-09-10 16:22 ` David Hildenbrand
2019-09-11 9:23 ` Michael S. Tsirkin
2019-09-11 9:50 ` David Hildenbrand [this message]
2019-09-10 17:52 ` Michal Hocko
2019-09-10 18:00 ` Michal Hocko
2019-09-10 20:37 ` Alexander Duyck
2019-09-10 21:23 ` Alexander Duyck
2019-09-11 11:36 ` Michal Hocko
2019-09-11 11:47 ` David Hildenbrand
2019-09-11 12:08 ` Michael S. Tsirkin
2019-09-11 12:19 ` Michal Hocko
2019-09-11 12:25 ` Michal Hocko
2019-09-11 12:42 ` David Hildenbrand
2019-09-11 12:54 ` Michal Hocko
2019-09-11 13:03 ` David Hildenbrand
2019-09-11 13:20 ` Michal Hocko
2019-09-11 13:51 ` Michal Hocko
2019-09-11 16:09 ` David Hildenbrand
2019-09-12 7:16 ` Michal Hocko
2019-09-12 7:47 ` David Hildenbrand
2019-09-12 9:26 ` Michal Hocko
2019-09-12 12:00 ` Nitesh Narayan Lal
2019-09-11 14:03 ` Nitesh Narayan Lal
2019-09-11 16:02 ` David Hildenbrand
2019-09-11 13:19 ` Nitesh Narayan Lal
2019-09-11 12:55 ` Nitesh Narayan Lal
2019-09-11 15:12 ` Alexander Duyck
2019-09-12 9:19 ` Michal Hocko
2019-09-12 10:24 ` Kirill A. Shutemov
2019-09-12 11:11 ` Michal Hocko
2019-09-12 15:42 ` Alexander Duyck
2019-09-12 16:35 ` Mel Gorman
2019-09-12 17:48 ` Alexander Duyck
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