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Date:   Wed, 29 Nov 2017 16:13:53 +0100
From:   Rasmus Villemoes <>
To:     Michal Hocko <>, <>
CC:     Khalid Aziz <>,
        Michael Ellerman <>,
        Andrew Morton <>,
        Russell King - ARM Linux <>,
        Andrea Arcangeli <>, <>,
        LKML <>,
        <>, Florian Weimer <>,
        John Hubbard <>,
        Abdul Haleem <>,
        Joel Stanley <>,
        Kees Cook <>,
        Michal Hocko <>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

On 2017-11-29 15:42, Michal Hocko wrote:
> The first patch introduced MAP_FIXED_SAFE which enforces the given
> address but unlike MAP_FIXED it fails with ENOMEM if the given range
> conflicts with an existing one.

[s/ENOMEM/EEXIST/, as it seems you also did in the actual patch and

>The flag is introduced as a completely
> new one rather than a MAP_FIXED extension because of the backward
> compatibility. We really want a never-clobber semantic even on older
> kernels which do not recognize the flag. Unfortunately mmap sucks wrt.
> flags evaluation because we do not EINVAL on unknown flags. On those
> kernels we would simply use the traditional hint based semantic so the
> caller can still get a different address (which sucks) but at least not
> silently corrupt an existing mapping. I do not see a good way around
> that.

I think it would be nice if this rationale was in the 1/2 changelog,
along with the hint about what userspace that wants to be compatible
with old kernels will have to do (namely, check that it got what it
requested) - which I see you did put in the man page.

Rasmus Villemoes
Software Developer
Prevas A/S
Hedeager 3
DK-8200 Aarhus N
+45 51210274

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