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

[Ira Weiny <ira.weiny@intel.com>]

On Thu, Feb 14, 2019 at 01:12:31PM -0700, Jason Gunthorpe wrote:
> 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.

I'm not against that.  Sorry if I came across that way.  For this series I
agree we should make it consistent.

> 
> > > 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.

I don't understand?  What is the other _user_ limit (other than
RLIMIT_MEMLOCK)?

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

Understood. :-/

Ira

> 
> Jason