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From:   Alexander Duyck <alexander.duyck@gmail.com>
Date:   Tue, 10 Sep 2019 07:42:43 -0700
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Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \
To:     Michal Hocko <mhocko@kernel.org>
Cc:     virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        David Hildenbrand <david@redhat.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>
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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.