linux-kernel - Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn

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Date:   Mon, 26 Feb 2018 08:49:20 +0100
From:   Mathieu Malaterre <malat@...ian.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        LKML <linux-kernel@...r.kernel.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jiri Slaby <jslaby@...e.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid

On Mon, Feb 26, 2018 at 7:32 AM, Christophe LEROY
<christophe.leroy@....fr> wrote:
>
>
> Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit :
>>
>> Rewrite comparison since all values compared are of type `unsigned long`.
>>
>> Fix a warning (treated as error in W=1):
>>
>>    CC      arch/powerpc/kernel/irq.o
>> In file included from ./include/linux/bug.h:5:0,
>>                   from ./include/linux/cpumask.h:13,
>>                   from ./include/linux/smp.h:13,
>>                   from ./include/linux/kernel_stat.h:5,
>>                   from arch/powerpc/kernel/irq.c:35:
>> ./include/linux/dma-mapping.h: In function ‘dma_map_resource’:
>> ./arch/powerpc/include/asm/page.h:129:32: error: comparison of unsigned
>> expression >= 0 is always true [-Werror=type-limits]
>>   #define pfn_valid(pfn)  ((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
>>                                  ^
>> Suggested-by: Segher Boessenkool <segher@...nel.crashing.org>
>> Signed-off-by: Mathieu Malaterre <malat@...ian.org>
>> ---
>>   arch/powerpc/include/asm/page.h | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/include/asm/page.h
>> b/arch/powerpc/include/asm/page.h
>> index 8da5d4c1cab2..19dea64e7ed2 100644
>> --- a/arch/powerpc/include/asm/page.h
>> +++ b/arch/powerpc/include/asm/page.h
>> @@ -126,7 +126,8 @@ extern long long virt_phys_offset;
>>     #ifdef CONFIG_FLATMEM
>>   #define ARCH_PFN_OFFSET               ((unsigned long)(MEMORY_START >>
>> PAGE_SHIFT))
>> -#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 ?

I assumed that normal unsigned integers modulo would make the test
fail. But for some particular value of max_mapnr the two are indeed
not equivalent.

> Christophe
>
>
>>   #endif
>>     #define virt_to_pfn(kaddr)  (__pa(kaddr) >> PAGE_SHIFT)
>>
>