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

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Message-ID: <20190214201231.GC1739@ziepe.ca>
Date:   Thu, 14 Feb 2019 13:12:31 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
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 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