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

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Date:   Mon, 14 Jan 2019 10:55:20 +0100
From:   Jan Kara <jack@...e.cz>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, Jan Kara <jack@...e.cz>,
        KVM list <kvm@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        Jason Wang <jasowang@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Qemu Developers <qemu-devel@...gnu.org>,
        virtualization@...ts.linux-foundation.org,
        adilger kernel <adilger.kernel@...ger.ca>,
        Ross Zwisler <zwisler@...nel.org>,
        dave jiang <dave.jiang@...el.com>,
        darrick wong <darrick.wong@...cle.com>,
        vishal l verma <vishal.l.verma@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        jmoyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>, lcapitulino@...hat.com,
        Kevin Wolf <kwolf@...hat.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device

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@...e.com>
SUSE Labs, CR