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From: Dan Williams <dan.j.williams@intel.com>
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>,
	linux-nvdimm <linux-nvdimm@ml01.01.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	virtualization@lists.linux-foundation.org,
	Linux ACPI <linux-acpi@vger.kernel.org>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>, Jan Kara <jack@suse.cz>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Rik van Riel <riel@surriel.com>,
	Nitesh Narayan Lal <nilal@redhat.com>,
	Kevin Wolf <kwolf@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ross Zwisler <zwisler@kernel.org>,
	vishal l verma <vishal.l.verma@intel.com>,
	dave jiang <dave.jiang@intel.com>,
	David Hildenbrand <david@redhat.com>, jmoyer <jmoyer@redhat.com>,
	xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	lcapitulino@redhat.com, Igor Mammedov <imammedo@redhat.com>,
	Eric Blake <eblake@redhat.com>, "Theodore Ts'o" <tytso@mit.edu>,
	adilger kernel <adilger.kernel@dilger.ca>,
	darrick wong <darrick.wong@oracle.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device
Date: Mon, 14 Jan 2019 13:35:57 -0800
Message-ID: <CAPcyv4jtPcLV-s0sKNHwwk0ug7GLBV6699dpm1h3r2xSo879dg@mail.gmail.com> (raw)
Message-ID: <20190114213557.51nRRVQSuMTZAc1aQrflDEiE0jaHSQBXvoifhfXeOnc@z> (raw)
In-Reply-To: <20190114212501.GG4205@dastard>

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.

  parent reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-09 14:47 Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 1/5] libnvdimm: nd_region flush callback support Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 2/5] virtio-pmem: Add virtio pmem driver Pankaj Gupta
2019-01-14 15:54   ` Michael S. Tsirkin
2019-01-14 15:54     ` Michael S. Tsirkin
     [not found]     ` <20190114105314-mutt-send-email-mst-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2019-01-15  6:33       ` Pankaj Gupta
2019-01-15  6:33         ` Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 3/5] libnvdimm: add nd_region buffered dax_dev flag Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 4/5] ext4: disable map_sync for virtio pmem Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 5/5] xfs: " Pankaj Gupta
2019-01-09 14:47   ` Pankaj Gupta
2019-01-09 16:26   ` Darrick J. Wong
2019-01-09 18:08     ` Pankaj Gupta
     [not found] ` <20190109144736.17452-1-pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-10  1:26   ` [PATCH v3 0/5] kvm "virtio pmem" device Dave Chinner
2019-01-10  1:26     ` Dave Chinner
2019-01-10  2:40     ` Rik van Riel
2019-01-10  2:40       ` Rik van Riel
2019-01-10 10:17     ` Jan Kara
2019-01-10 10:17       ` Jan Kara
     [not found]       ` <20190110101757.GC15790-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org>
2019-01-13  1:38         ` Pankaj Gupta
2019-01-13  1:38           ` Pankaj Gupta
     [not found]           ` <1354249849.63357171.1547343519970.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-13  1:43             ` Dan Williams
2019-01-13  1:43               ` Dan Williams
     [not found]               ` <CAPcyv4hwcgTUpgNCefCGu4DvgkYBp5b=f+hJ+FC=s5APYKoycg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-01-13  2:17                 ` [Qemu-devel] " Pankaj Gupta
2019-01-13  2:17                   ` Pankaj Gupta
     [not found]                   ` <540171952.63371441.1547345866585.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-14  9:55                     ` Jan Kara
2019-01-14  9:55                       ` Jan Kara
     [not found]                       ` <20190114095520.GC13316-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org>
2019-01-14 10:16                         ` Pankaj Gupta
2019-01-14 10:16                           ` Pankaj Gupta
2019-01-11  7:45     ` Pankaj Gupta
2019-01-11  7:45       ` Pankaj Gupta
     [not found]       ` <1326478078.61913951.1547192704870.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-13 23:29         ` Dave Chinner
2019-01-13 23:29           ` Dave Chinner
2019-01-13 23:38           ` Matthew Wilcox
2019-01-13 23:38             ` Matthew Wilcox
     [not found]             ` <20190113233820.GX6310-PfSpb0PWhxZc2C7mugBRk2EX/6BAtgUQ@public.gmane.org>
2019-01-14  2:50               ` Dave Chinner
2019-01-14  2:50                 ` Dave Chinner
2019-01-14  7:15               ` Pankaj Gupta
2019-01-14  7:15                 ` Pankaj Gupta
     [not found]                 ` <942065073.64011540.1547450140670.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-14 21:25                   ` Dave Chinner
2019-01-14 21:25                     ` Dave Chinner
2019-01-14 21:35                     ` Dan Williams [this message]
2019-01-14 21:35                       ` Dan Williams
     [not found]                       ` <CAPcyv4jtPcLV-s0sKNHwwk0ug7GLBV6699dpm1h3r2xSo879dg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-01-14 22:21                         ` Dave Chinner
2019-01-14 22:21                           ` Dave Chinner
2019-01-15  2:19                           ` Michael S. Tsirkin
2019-01-15  2:19                             ` Michael S. Tsirkin
     [not found]                             ` <20190114205031-mutt-send-email-mst-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2019-01-15  5:37                               ` [Qemu-devel] " Pankaj Gupta
2019-01-15  5:37                                 ` Pankaj Gupta
2019-01-15  5:35                           ` Pankaj Gupta
2019-01-15  5:35                             ` Pankaj Gupta
     [not found]                             ` <1684638419.64320214.1547530506805.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-15 20:42                               ` Dave Chinner
2019-01-15 20:42                                 ` Dave Chinner
2019-02-04 22:56 ` security implications of caching with virtio pmem (was Re: [PATCH v3 0/5] kvm "virtio pmem" device) Michael S. Tsirkin
2019-02-05  7:29   ` [Qemu-devel] " Pankaj Gupta
2019-02-06 14:00   ` David Hildenbrand
2019-02-06 18:01     ` Michael S. Tsirkin
2019-02-11  7:29   ` [Qemu-devel] " Pankaj Gupta
2019-02-11 22:29     ` Dave Chinner
2019-02-11 22:58       ` David Hildenbrand
2019-02-11 23:07         ` Michael S. Tsirkin
  -- strict thread matches above, loose matches on Subject: below --
2019-01-09 13:50 [PATCH v3 0/5] kvm "virtio pmem" device Pankaj Gupta

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