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
[off-list ref] wrote:

Le 25/02/2018 =C3=A0 18:22, Mathieu Malaterre a =C3=A9crit :

quoted

Rewrite comparison since all values compared are of type `unsigned long`=

.

quoted

Fix a warning (treated as error in W=3D1):

   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 =E2=80=98dma_map_resource=E2=

=80=99:

quoted

./arch/powerpc/include/asm/page.h:129:32: error: comparison of unsigned
expression >=3D 0 is always true [-Werror=3Dtype-limits]
  #define pfn_valid(pfn)  ((pfn) >=3D ARCH_PFN_OFFSET && (pfn) < max_map=

nr)

quoted

                                 ^
Suggested-by: Segher Boessenkool <redacted>
Signed-off-by: Mathieu Malaterre <redacted>
---
  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) >=3D 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

quoted

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