Date: Mon, 14 Jan 2019 10:55:20 +0100
From: Jan Kara <jack@suse.cz>
To: Pankaj Gupta <pagupta@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>,
        KVM list <kvm@vger.kernel.org>,
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Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device
Message-ID: <20190114095520.GC13316@quack2.suse.cz>
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On Sat 12-01-19 21:17:46, Pankaj Gupta wrote:
> > > > Right. Thinking about this I would be more concerned about the fact that
> > > > guest can effectively pin amount of host's page cache upto size of the
> > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there some
> > > > QEMU
> > > > magic that avoids this?
> > >
> > > Yes, guest will pin these host page cache pages using 'get_user_pages' by
> > > elevating the page reference count. But these pages can be reclaimed by
> > > host
> > > at any time when there is memory pressure.
> > 
> > Wait, how can the guest pin the host pages? I would expect this to
> > happen only when using vfio and device assignment. Otherwise, no the
> > host can't reclaim a pinned page, that's the whole point of a pin to
> > prevent the mm from reclaiming ownership.
> 
> yes. You are right I just used the pin word but it does not actually pin pages 
> permanently. I had gone through the discussion on existing problems with 
> get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP 
> pin pages so I also used the word 'pin'. But guest does not permanently pin 
> these pages and these pages can be reclaimed by host.

OK, then I was just confused how virtio-pmem is going to work. Thanks for
explanation! So can I imagine this as guest mmaping the host file and
providing the mapped range as "NVDIMM pages" to the kernel inside the
guest? Or is it more complex?

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR