linux-kernel - Re: [PATCH v3 1/4] mm: introduce get_user_pages

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Date:   Fri, 1 Dec 2017 09:02:04 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm

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?

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

Jason