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

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Date:   Thu, 30 Nov 2017 10:03:26 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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>
Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm

On Thu, Nov 30, 2017 at 9:42 AM, Michal Hocko <mhocko@...nel.org> wrote:
>
> On Thu 30-11-17 08:39:51, Dan Williams wrote:
> > On Thu, Nov 30, 2017 at 1:53 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > > On Wed 29-11-17 10:05:35, Dan Williams 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).
> > >
> > > One thing is not clear to me. Who is allowed to pin pages for ever?
> > > Is it possible to pin LRU pages that way as well? If yes then there
> > > absolutely has to be a limit for that. Sorry I could have studied the
> > > code much more but from a quick glance it seems to me that this is not
> > > limited to dax (or non-LRU in general) pages.
> >
> > I would turn this question around. "who can not tolerate a page being
> > pinned forever?".
>
> Any struct page on the movable zone or anything that is living on the
> LRU list because such a memory is unreclaimable.
>
> > In the case of filesytem-dax a page is
> > one-in-the-same object as a filesystem-block, and a filesystem expects
> > that its operations will not be blocked indefinitely. LRU pages can
> > continue to be pinned indefinitely because operations can continue
> > around the pinned page, i.e. every agent, save for the dma agent,
> > drops their reference to the page and its tolerable that the final
> > put_page() never arrives.
>
> I do not understand. Are you saying that a user triggered IO can pin LRU
> pages indefinitely. This would be _really_ wrong. It would be basically
> an mlock without any limit. So I must be misreading you here

You're not misreading. See ib_umem_get() for example, it pins pages in
response to the userspace library call ibv_reg_mr() (memory
registration), and will not release those pages unless/until a call to
ibv_dereg_mr() is made. The current plan to fix this is to create
something like a ibv_reg_mr_lease() call that registers the memory
with an F_SETLEASE semantic so that the kernel can notify userspace
that a memory registration is being forcibly revoked by the kernel. A
previous attempt at something like this was the proposed MAP_DIRECT
mmap flag [1].

[1]: https://lists.01.org/pipermail/linux-nvdimm/2017-October/012815.html