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From: Dave Chinner <david@fromorbit.com>
To: Pankaj Gupta <pagupta@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	qemu-devel@nongnu.org, linux-nvdimm@ml01.01.org,
	linux-fsdevel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-acpi@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, jack@suse.cz, stefanha@redhat.com,
	dan j williams <dan.j.williams@intel.com>,
	riel@surriel.com, nilal@redhat.com, kwolf@redhat.com,
	pbonzini@redhat.com, zwisler@kernel.org,
	vishal l verma <vishal.l.verma@intel.com>,
	dave jiang <dave.jiang@intel.com>,
	david@redhat.com, jmoyer@redhat.com,
	xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
	hch@infradead.org, mst@redhat.com, jasowang@redhat.com,
	lcapitulino@redhat.com, imammedo@redhat.com, eblake@redhat.com,
	tytso@mit.edu, adilger kernel <adilger.kernel@dilger.ca>,
	darrick wong <darrick.wong@oracle.com>,
	rjw@rjwysocki.net
Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device
Date: Tue, 15 Jan 2019 08:25:01 +1100
Message-ID: <20190114212501.GG4205@dastard> (raw)
Message-ID: <20190114212501.ttaSq8CgyrgNBQZwsGKQ2S-3tBIATUDzyWmJDMrSLPM@z> (raw)
In-Reply-To: <942065073.64011540.1547450140670.JavaMail.zimbra@redhat.com>

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....

> 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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  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 [this message]
2019-01-14 21:25                     ` Dave Chinner
2019-01-14 21:35                     ` Dan Williams
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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