Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address
From: Michael S. Tsirkin
Date: Thu Dec 13 2018 - 16:58:11 EST
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?
I'd suggest leave vmap alone and find ways to speed up accesses
that can fault.