linux-kernel - Re: [PATCH v2 1/4] mm: introduce get_user_pages

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Date:   Tue, 21 Nov 2017 15:26:26 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>
Subject: Re: [PATCH v2 1/4] mm: introduce get_user_pages_longterm

On Tue, Nov 21, 2017 at 3:05 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
> On Tue, 14 Nov 2017 11:56:34 -0800 Dan Williams <dan.j.williams@...el.com> wrote:
>
> > Until there is a solution to the dma-to-dax vs truncate problem it is
> > not safe to allow long standing memory registrations against
> > filesytem-dax vmas. Device-dax vmas do not have this problem and are
> > explicitly allowed.
> >
> > This is temporary until a "memory registration with layout-lease"
> > mechanism can be implemented for the affected sub-systems (RDMA and
> > V4L2).
>
> Sounds like that will be unpleasant.  Do we really need it to be that
> complex?  Can we get away with simply failing the get_user_pages()
> request?  Or are there significant usecases for RDMA and V4L to play
> with DAX memory?

V4L plus DAX is indeed dubious, but RDMA to persistent memory is
something the RDMA community is interested in supporting [1].

[1]: http://www.snia.org/sites/default/files/SDC15_presentations/persistant_mem/ChetDouglas_RDMA_with_PM.pdf