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;
Date:   Tue, 10 Sep 2019 17:18:18 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To:     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>,
        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>
Subject: Re: [virtio-dev] Re: [PATCH v9 0/8] stg mail -e --version=v9 \
Message-ID: <20190910161818.GF2797@work-vm>
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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKgT0UdB4qp3vFGrYEs=FwSXKpBEQ7zo7DV55nJRO2C-KCEOrw@mail.gmail.com>
User-Agent: Mutt/1.12.1 (2019-06-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

* 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

> ---------------------------------------------------------------------
> To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
> For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK