linux-ext4 - Re: [PATCH v3 0/5] kvm "virtio pmem" device

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Message-ID: <942065073.64011540.1547450140670.JavaMail.zimbra@redhat.com>
Date:   Mon, 14 Jan 2019 02:15:40 -0500 (EST)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, qemu-devel@...gnu.org,
        linux-nvdimm@...1.01.org, linux-fsdevel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-acpi@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, jack@...e.cz, stefanha@...hat.com,
        dan j williams <dan.j.williams@...el.com>, riel@...riel.com,
        nilal@...hat.com, kwolf@...hat.com, pbonzini@...hat.com,
        zwisler@...nel.org, vishal l verma <vishal.l.verma@...el.com>,
        dave jiang <dave.jiang@...el.com>, david@...hat.com,
        jmoyer@...hat.com,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        hch@...radead.org, mst@...hat.com, jasowang@...hat.com,
        lcapitulino@...hat.com, imammedo@...hat.com, eblake@...hat.com,
        tytso@....edu, adilger kernel <adilger.kernel@...ger.ca>,
        darrick wong <darrick.wong@...cle.com>, rjw@...ysocki.net
Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device


> > 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. 
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.  

Thanks,
Pankaj