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Message-ID: <CAPcyv4jtPcLV-s0sKNHwwk0ug7GLBV6699dpm1h3r2xSo879dg@mail.gmail.com> Date: Mon, 14 Jan 2019 13:35:57 -0800 From: Dan Williams <dan.j.williams@...el.com> To: Dave Chinner <david@...morbit.com> Cc: Pankaj Gupta <pagupta@...hat.com>, Matthew Wilcox <willy@...radead.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, KVM list <kvm@...r.kernel.org>, Qemu Developers <qemu-devel@...gnu.org>, linux-nvdimm <linux-nvdimm@...1.01.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, virtualization@...ts.linux-foundation.org, Linux ACPI <linux-acpi@...r.kernel.org>, linux-ext4 <linux-ext4@...r.kernel.org>, linux-xfs <linux-xfs@...r.kernel.org>, Jan Kara <jack@...e.cz>, Stefan Hajnoczi <stefanha@...hat.com>, Rik van Riel <riel@...riel.com>, Nitesh Narayan Lal <nilal@...hat.com>, Kevin Wolf <kwolf@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, Ross Zwisler <zwisler@...nel.org>, vishal l verma <vishal.l.verma@...el.com>, dave jiang <dave.jiang@...el.com>, David Hildenbrand <david@...hat.com>, jmoyer <jmoyer@...hat.com>, xiaoguangrong eric <xiaoguangrong.eric@...il.com>, Christoph Hellwig <hch@...radead.org>, "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, lcapitulino@...hat.com, Igor Mammedov <imammedo@...hat.com>, Eric Blake <eblake@...hat.com>, "Theodore Ts'o" <tytso@....edu>, adilger kernel <adilger.kernel@...ger.ca>, darrick wong <darrick.wong@...cle.com>, "Rafael J. Wysocki" <rjw@...ysocki.net> Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device On Mon, Jan 14, 2019 at 1:25 PM Dave Chinner <david@...morbit.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.
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