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    SubjectRe: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \
    * 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.

    Dave

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    --
    Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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