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From:   Mathieu Malaterre <malat@debian.org>
Date:   Fri, 2 Mar 2018 20:54:27 +0100
Message-ID: <CA+7wUsz7srOXwehLjuSn_SEgCUkGxu9bq6aOw5AeOaiiipBNtg@mail.gmail.com>
Subject: Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Christophe LEROY <christophe.leroy@c-s.fr>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Paul Mackerras <paulus@samba.org>,
        LKML <linux-kernel@vger.kernel.org>, Jiri Slaby <jslaby@suse.com>
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On Mon, Feb 26, 2018 at 9:46 AM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Mon, Feb 26, 2018 at 07:32:03AM +0100, Christophe LEROY wrote:
>> Le 25/02/2018 =C3=A0 18:22, Mathieu Malaterre a =C3=A9crit :
>> >-#define pfn_valid(pfn)              ((pfn) >=3D ARCH_PFN_OFFSET && (pf=
n) <
>> >max_mapnr)
>> >+#define pfn_valid(pfn) \
>> >+            (((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET)=
)
>>
>> What will happen when ARCH_PFN_OFFSET is not nul and pfn is lower than
>> ARCH_PFN_OFFSET ?
>
> It will work fine.
>
> Say you are asking for  a <=3D x < b  so (in actual integers, no overflow=
)
> that is  0 <=3D x-a < b-a  and you also assume x-a overflows, so that we
> are actually comparing  x-a+M < b-a  with M =3D 2**32 or such (the maximu=
m
> value in the unsigned integer type plus one).  This comparison is
> obviously always false.
>
> (It also works if b < a btw).
>
>
Thanks Segher !

Christophe does that clarify things or do you want me to update the
commit message ?