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Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE
To:     Michal Hocko <mhocko@kernel.org>, <linux-api@vger.kernel.org>
CC:     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>, Florian Weimer <fweimer@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
        Joel Stanley <joel@jms.id.au>,
        Kees Cook <keescook@chromium.org>,
        Michal Hocko <mhocko@suse.com>
References: <20171129144219.22867-1-mhocko@kernel.org>
From:   Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Message-ID: <b154b794-7a8b-995e-0954-9234b9446b31@prevas.dk>
Date:   Wed, 29 Nov 2017 16:13:53 +0100
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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
changelog]

>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
rasmus.villemoes@prevas.dk
www.prevas.dk

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