Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jason Wang <jasowang@redhat.com>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address
Date: Thu, 13 Dec 2018 16:58:04 -0500	[thread overview]
Message-ID: <20181213165731-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20181213211840.GC18692@char.us.oracle.com>

On Thu, Dec 13, 2018 at 04:18:40PM -0500, Konrad Rzeszutek Wilk wrote:
> .giant snip..
> > > +	npinned = get_user_pages_fast(uaddr, npages, write, pages);
> > > +	if (npinned != npages)
> > > +		goto err;
> > > +
> > 
> > As I said I have doubts about the whole approach, but this
> > implementation in particular isn't a good idea
> > as it keeps the page around forever.
> > So no THP, no NUMA rebalancing, userspace-controlled
> > amount of memory locked up and not accounted for.
> > 
> > Don't get me wrong it's a great patch in an ideal world.
> > But then in an ideal world no barriers smap etc are necessary at all.
> 
> So .. suggestions on how this could be accepted? As in other ways
> where we still get vmap and the issues you mentioned are not troubling you?
> 
> Thanks!

I'd suggest leave vmap alone and find ways to speed up accesses
that can fault.

-- 
MST