Re: [PATCH v3 0/5] kvm "virtio pmem" device

[Dave Chinner]

On Mon, Jan 14, 2019 at 01:35:57PM -0800, Dan Williams wrote:
> On Mon, Jan 14, 2019 at 1:25 PM Dave Chinner <david@xxxxxxxxxxxxx> 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.

So at what point do we draw the line?

We're allowing writable DAX mappings, but as I've pointed out that
means we are going to be allowing  a potential information leak via
files with shared extents to be directly mapped and written to.

But we won't allow useful admin operations that allow better
management of host side storage space similar to how normal image
files are used by guests because it's an information leak vector?

That's splitting some really fine hairs there...

> > > 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.

"if"?

I expect it to be "when", not if. Expect the worst, plan for it now.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx