From: Michal Hocko <mhocko@kernel.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-api@vger.kernel.org, Khalid Aziz <khalid.aziz@oracle.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Andrew Morton <akpm@linux-foundation.org>,
Russell King - ARM Linux <linux@armlinux.org.uk>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
linux-arch@vger.kernel.org,
Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
Joel Stanley <joel@jms.id.au>, Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH 0/2] mm: introduce MAP_FIXED_SAFE
Date: Wed, 22 Nov 2017 14:12:45 +0100 [thread overview]
Message-ID: <20171122131245.fpqtipwdxzuaj6gl@dhcp22.suse.cz> (raw)
In-Reply-To: <9a02b37c-978a-48ef-0b22-b1e4cbb9a704@nvidia.com>
On Tue 21-11-17 17:48:31, John Hubbard wrote:
[...]
> Hi Michal,
>
> Yes, it really is useful for user space. I'll use CUDA as an example, but I
> think anything that enforces a uniform virtual addressing scheme across CPUs
> and devices, probably has to do something eerily similar. CUDA does this:
>
> a) Searches /proc/<pid>/maps for a "suitable" region of available VA space.
> "Suitable" generally means it has to have a base address within a certain
> limited range (a particular device model might have odd limitations, for
> example), it has to be large enough, and alignment has to be large enough
> (again, various devices may have constraints that lead us to do this).
>
> This is of course subject to races with other threads in the process.
>
> Let's say it finds a region starting at va.
>
> b) Next it does:
> p = mmap(va, ...)
>
> *without* setting MAP_FIXED, of course (so va is just a hint), to attempt to
> safely reserve that region. If p != va, then in most cases, this is a failure
> (almost certainly due to another thread getting a mapping from that region
> before we did), and so this layer now has to call munmap(), before returning
> a "failure: retry" to upper layers.
>
> IMPROVEMENT: --> if instead, we could call this:
>
> p = mmap(va, ... MAP_FIXED_NO_CLOBBER ...)
>
> , then we could skip the munmap() call upon failure. This is a small thing,
> but it is useful here. (Thanks to Piotr Jaroszynski and Mark Hairgrove
> for helping me get that detail exactly right, btw.)
>
> c) After that, CUDA suballocates from p, via:
>
> q = mmap(sub_region_start, ... MAP_FIXED ...)
>
> Interestingly enough, "freeing" is also done via MAP_FIXED, and setting PROT_NONE
> to the subregion. Anyway, I just included (c) for general interest.
OK, I will add this to the changelog. This is basically the "Atomic
address range probing in the multithreaded programs" I've had in the
cover letter.
> I expect that as we continue working on the open source compute software stack,
> this new capability will be useful there, too.
>
> Oh, and on the naming, when I described how your implementation worked (without
> naming it) to Piotr, he said, "oh, something like map-fixed-no-clobber?". So I
> think my miniature sociology naming data point here can bolster the case ever so
> slightly for calling it MAP_FIXED_NO_CLOBBER. haha. :)
I will be probably stubborn and go with a shorter name I have currently.
I am not very fond-of-very-long-names.
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-11-22 13:12 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-16 10:18 (unknown), Michal Hocko
2017-11-16 10:18 ` [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE Michal Hocko
2017-11-17 0:27 ` Kees Cook
[not found] ` <CAGXu5jKssQCcYcZujvQeFy5LTzhXSW=f-a0riB=4+caT1i38BQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-17 19:12 ` Matthew Wilcox
2017-11-20 8:43 ` Michal Hocko
2017-11-17 7:30 ` Florian Weimer
2017-11-20 8:55 ` Michal Hocko
2017-11-20 9:10 ` Florian Weimer
[not found] ` <37a6e9ba-e0df-b65f-d5ef-871c25b5cb87-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-11-20 9:33 ` Michal Hocko
2017-11-20 9:45 ` Florian Weimer
2017-11-17 8:37 ` John Hubbard
2017-11-20 9:02 ` Michal Hocko
2017-11-16 10:19 ` [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map Michal Hocko
2017-11-17 0:30 ` Kees Cook
[not found] ` <20171116101900.13621-1-mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2017-11-16 12:14 ` [RFC PATCH 0/2] mm: introduce MAP_FIXED_SAFE Michal Hocko
[not found] ` <20171116121438.6vegs4wiahod3byl-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-11-17 8:45 ` John Hubbard
2017-11-20 9:05 ` Michal Hocko
2017-11-22 1:48 ` John Hubbard
2017-11-22 13:12 ` Michal Hocko [this message]
2017-11-22 13:20 ` Vlastimil Babka
2017-11-24 8:54 ` Michal Hocko
2017-11-27 15:51 ` Khalid Aziz
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