Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid

[christophe leroy]

Le 02/03/2018 Ã 20:54, Mathieu Malaterre a ÃcritÂ:
> On Mon, Feb 26, 2018 at 9:46 AM, Segher Boessenkool
> <segher@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, Feb 26, 2018 at 07:32:03AM +0100, Christophe LEROY wrote:
> > > Le 25/02/2018 Ã 18:22, Mathieu Malaterre a Ãcrit :
> > > > -#define pfn_valid(pfn)              ((pfn) >= ARCH_PFN_OFFSET && (pfn) <
> > > > 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 <= x < b  so (in actual integers, no overflow)
> > that is  0 <= x-a < b-a  and you also assume x-a overflows, so that we
> > are actually comparing  x-a+M < b-a  with M = 2**32 or such (the maximum
> > 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 ?

No it is fine for me.

Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxx>

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