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. > > 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.
