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From: Dan Williams <dan.j.williams@intel.com>
Date: Fri, 1 Dec 2017 08:29:53 -0800
Message-ID: <CAPcyv4hvk8rfV_=5EX3QPFLZ=LB4=hWG5h4Z42koNYim9DB3FQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        linux-rdma <linux-rdma@vger.kernel.org>
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On Fri, Dec 1, 2017 at 8:02 AM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Dec 01, 2017 at 11:12:18AM +0100, Michal Hocko wrote:
> > On Thu 30-11-17 12:01:17, Jason Gunthorpe wrote:
> > > On Thu, Nov 30, 2017 at 10:32:42AM -0800, Dan Williams wrote:
> > > > > Who and how many LRU pages can pin that way and how do you prevent nasty
> > > > > users to DoS systems this way?
> > > >
> > > > I assume this is something the RDMA community has had to contend with?
> > > > I'm not an RDMA person, I'm just here to fix dax.
> > >
> > > The RDMA implementation respects the mlock rlimit
> >
> > OK, so then I am kind of lost in why do we need a special g-u-p variant.
> > The documentation doesn't say and quite contrary it assumes that the
> > caller knows what he is doing. This cannot be the right approach.
>
> I thought it was because get_user_pages_longterm is supposed to fail
> on DAX mappings?

Correct, the rlimit checks are a separate issue,
get_user_pages_longterm is only there to avoid open coding vma lookup
and vma_is_fsdax() checks in multiple code paths.

> And maybe we should think about moving the rlimit accounting into this
> new function too someday?

DAX pages are not accounted in any rlimit because they are statically
allocated reserved memory regions.