> -----Original Message----- > From: Michal Hocko [mailto:mho...@kernel.org] > Sent: Wednesday, November 30, 2016 12:17 PM > To: Yuriy Kolerov <yuriy.kole...@synopsys.com> > Cc: linux-snps-...@lists.infradead.org; vineet.gup...@synopsys.com; > alexey.brod...@synopsys.com; linux-kernel@vger.kernel.org > Subject: Re: [RFC] ARC: mm: Restrict definition of pfn_valid() macro for > CONFIG_FLATMEM > > On Tue 29-11-16 18:29:06, Yuriy Kolerov wrote: > > Despite the fact that subtraction of unsigned integers is a defined > > behaviour however such operations can lead to unexpected results. Thus > > it is better to check both left and right boundaries to avoid > > potential bugs as it done in the generic page.h. > > Why and which code would use an out of range pfn? Why other arches do > not need to care?
Actually some arches do care about checking of both left and right boundaries (e.g. avr32, sparc, etc). The problem is that a value of pfn may be calculated incorrectly in some places of the kernel. E.g. not long ago I sent a patch which fixes truncation of the most significant byte in pfn/pte in some cases (in the kernel with PAE40, however it is not a FLATMEM case). So such situations can happens in the most unexpected places. > > > > Signed-off-by: Yuriy Kolerov <yuriy.kole...@synopsys.com> > > --- > > arch/arc/include/asm/page.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/arc/include/asm/page.h b/arch/arc/include/asm/page.h > > index 296c342..81cfc6c7 100644 > > --- a/arch/arc/include/asm/page.h > > +++ b/arch/arc/include/asm/page.h > > @@ -88,7 +88,7 @@ typedef pte_t * pgtable_t; > > #define ARCH_PFN_OFFSET > virt_to_pfn(CONFIG_LINUX_LINK_BASE) > > > > #ifdef CONFIG_FLATMEM > > -#define pfn_valid(pfn) (((pfn) - ARCH_PFN_OFFSET) < > max_mapnr) > > +#define pfn_valid(pfn) ((pfn) >= ARCH_PFN_OFFSET && > ((pfn) - ARCH_PFN_OFFSET) < max_mapnr) > > #endif > > > > /* > > -- > > 2.7.4 > > > > -- > Michal Hocko > SUSE Labs
