From: Dan Williams <dan.j.williams@intel.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: 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>
Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm
Date: Thu, 30 Nov 2017 10:03:26 -0800 [thread overview]
Message-ID: <CAPcyv4h5GUueqB-QhbWbn39SBPDE-rOte6UcmAHSWQdVyrF2Rw@mail.gmail.com> (raw)
In-Reply-To: <20171130174201.stbpuye4gu5rxwkm@dhcp22.suse.cz>
On Thu, Nov 30, 2017 at 9:42 AM, Michal Hocko <mhocko@kernel.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@kernel.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
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next prev parent reply other threads:[~2017-11-30 18:03 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-29 18:05 [PATCH v3 0/4] introduce get_user_pages_longterm() Dan Williams
2017-11-29 18:05 ` [PATCH v3 1/4] mm: introduce get_user_pages_longterm Dan Williams
2017-11-30 9:53 ` Michal Hocko
2017-11-30 16:39 ` Dan Williams
2017-11-30 17:42 ` Michal Hocko
2017-11-30 18:03 ` Dan Williams [this message]
2017-11-30 18:17 ` Michal Hocko
2017-11-30 18:32 ` Dan Williams
2017-11-30 19:01 ` Jason Gunthorpe
2017-12-01 10:12 ` Michal Hocko
2017-12-01 16:02 ` Jason Gunthorpe
2017-12-01 16:29 ` Dan Williams
2017-12-01 16:31 ` Jason Gunthorpe
2017-12-04 9:31 ` Michal Hocko
2017-12-04 17:01 ` Dan Williams
2017-11-29 18:05 ` [PATCH v3 2/4] mm: fail get_vaddr_frames() for filesystem-dax mappings Dan Williams
2017-11-29 18:05 ` [PATCH v3 3/4] [media] v4l2: disable filesystem-dax mapping support Dan Williams
2017-11-29 18:05 ` [PATCH v3 4/4] IB/core: disable memory registration of fileystem-dax vmas Dan Williams
2017-11-29 18:36 ` Jason Gunthorpe
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