Re: [PATCH 0/5] use pinned_vm instead of locked_vm to account pinned pages

[Jason Gunthorpe]

On Thu, Feb 14, 2019 at 11:33:53AM -0800, Ira Weiny wrote:

> > I think it had to do with double accounting pinned and mlocked pages
> > and thus delivering a lower than expected limit to userspace.
> > 
> > vfio has this bug, RDMA does not. RDMA has a bug where it can
> > overallocate locked memory, vfio doesn't.
> 
> Wouldn't vfio also be able to overallocate if the user had RDMA pinned pages?

Yes
 
> I think the problem is that if the user calls mlock on a large range then both
> vfio and RDMA could potentially overallocate even with this fix.  This was your
> initial email to Daniel, I think...  And Alex's concern.

Here are the possibilities
- mlock and pin on the same pages - RDMA respects the limit, VFIO halfs it.
- mlock and pin on different pages - RDMA doubles the limit, VFIO
  respects it
- VFIO and RDMA in the same process, the limit is halfed or doubled, depending.

IHMO we should make VFIO & RDMA the same, and then decide what to do
about case #2.

> > Really unclear how to fix this. The pinned/locked split with two
> > buckets may be the right way.
> 
> Are you suggesting that we have 2 user limits?

This is what RDMA has done since CL's patch.

It is very hard to fix as you need to track how many pages are mlocked
*AND* pinned.

Jason