linux-kernel - Re: [PATCH 0/5] use pinned_vm instead of locked

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Date:   Thu, 14 Feb 2019 13:46:51 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        akpm@...ux-foundation.org, dave@...olabs.net, jack@...e.cz,
        cl@...ux.com, linux-mm@...ck.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, paulus@...abs.org,
        benh@...nel.crashing.org, mpe@...erman.id.au, hao.wu@...el.com,
        atull@...nel.org, mdf@...nel.org, aik@...abs.ru
Subject: Re: [PATCH 0/5] use pinned_vm instead of locked_vm to account pinned
 pages

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