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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Mon, 14 Jan 2019 13:35:57 -0800
Message-ID: <CAPcyv4jtPcLV-s0sKNHwwk0ug7GLBV6699dpm1h3r2xSo879dg@mail.gmail.com>
Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device
To:     Dave Chinner <david@fromorbit.com>
Cc:     Pankaj Gupta <pagupta@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        Qemu Developers <qemu-devel@nongnu.org>,
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        linux-xfs <linux-xfs@vger.kernel.org>, Jan Kara <jack@suse.cz>,
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        Rik van Riel <riel@surriel.com>,
        Nitesh Narayan Lal <nilal@redhat.com>,
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        Paolo Bonzini <pbonzini@redhat.com>,
        Ross Zwisler <zwisler@kernel.org>,
        vishal l verma <vishal.l.verma@intel.com>,
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On Mon, Jan 14, 2019 at 1:25 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Mon, Jan 14, 2019 at 02:15:40AM -0500, Pankaj Gupta wrote:
> >
> > > > Until you have images (and hence host page cache) shared between
> > > > multiple guests. People will want to do this, because it means they
> > > > only need a single set of pages in host memory for executable
> > > > binaries rather than a set of pages per guest. Then you have
> > > > multiple guests being able to detect residency of the same set of
> > > > pages. If the guests can then, in any way, control eviction of the
> > > > pages from the host cache, then we have a guest-to-guest information
> > > > leak channel.
> > >
> > > I don't think we should ever be considering something that would allow a
> > > guest to evict page's from the host's pagecache [1].  The guest should
> > > be able to kick its own references to the host's pagecache out of its
> > > own pagecache, but not be able to influence whether the host or another
> > > guest has a read-only mapping cached.
> > >
> > > [1] Unless the guest is allowed to modify the host's file; obviously
> > > truncation, holepunching, etc are going to evict pages from the host's
> > > page cache.
> >
> > This is so correct. Guest does not not evict host page cache pages directly.
>
> They don't right now.
>
> But someone is going to end up asking for discard to work so that
> the guest can free unused space in the underlying spares image (i.e.
> make use of fstrim or mount -o discard) because they have workloads
> that have bursts of space usage and they need to trim the image
> files afterwards to keep their overall space usage under control.
>
> And then....

...we reject / push back on that patch citing the above concern.

> > In case of virtio-pmem & DAX, guest clears guest page cache exceptional entries.
> > Its solely decision of host to take action on the host page cache pages.
> >
> > In case of virtio-pmem, guest does not modify host file directly i.e don't
> > perform hole punch & truncation operation directly on host file.
>
> ... this will no longer be true, and the nuclear landmine in this
> driver interface will have been armed....

I agree with the need to be careful when / if explicit cache control
is added, but that's not the case today.