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Date:   Thu, 14 Feb 2019 13:12:31 -0700
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     Daniel Jordan <daniel.m.jordan@oracle.com>,
        akpm@linux-foundation.org, dave@stgolabs.net, jack@suse.cz,
        cl@linux.com, linux-mm@kvack.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-fpga@vger.kernel.org, linux-kernel@vger.kernel.org,
        alex.williamson@redhat.com, paulus@ozlabs.org,
        benh@kernel.crashing.org, mpe@ellerman.id.au, hao.wu@intel.com,
        atull@kernel.org, mdf@kernel.org, aik@ozlabs.ru
Subject: Re: [PATCH 0/5] use pinned_vm instead of locked_vm to account pinned
 pages
Message-ID: <20190214201231.GC1739@ziepe.ca>
References: <20190211224437.25267-1-daniel.m.jordan@oracle.com>
 <20190211225447.GN24692@ziepe.ca>
 <20190214015314.GB1151@iweiny-DESK2.sc.intel.com>
 <20190214060006.GE24692@ziepe.ca>
 <20190214193352.GA7512@iweiny-DESK2.sc.intel.com>
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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