Re: [PATCH next 11/14] bit: Strengthen compile-time tests in GENMASK() and BIT()

[David Laight <david.laight.linux@gmail.com>]

On Thu, 22 Jan 2026 09:11:53 +0800
kernel test robot <lkp@intel.com> wrote:

> Hi,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on next-20260120]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/david-laight-linux-gmail-com/overflow-Reduce-expansion-of-__type_max/20260122-013456
> base:   next-20260120
> patch link:    https://lore.kernel.org/r/20260121145731.3623-12-david.laight.linux%40gmail.com
> patch subject: [PATCH next 11/14] bit: Strengthen compile-time tests in GENMASK() and BIT()
> config: s390-randconfig-001-20260122 (https://download.01.org/0day-ci/archive/20260122/202601220829.MgTMeqqN-lkp@intel.com/config)
> compiler: s390-linux-gcc (GCC) 8.5.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260122/202601220829.MgTMeqqN-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202601220829.MgTMeqqN-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from arch/s390/include/asm/bug.h:60,
>                     from include/linux/bug.h:5,
>                     from include/linux/mmdebug.h:5,
>                     from arch/s390/include/asm/cmpxchg.h:11,
>                     from arch/s390/include/asm/atomic.h:16,
>                     from include/linux/atomic.h:7,
>                     from include/asm-generic/bitops/atomic.h:5,
>                     from arch/s390/include/asm/bitops.h:75,
>                     from include/linux/bitops.h:67,
>                     from include/linux/kernel.h:23,
>                     from net/core/page_pool.c:10:
>    In function 'netmem_clear_pp_magic',
>        inlined from 'page_pool_clear_pp_info' at net/core/page_pool.c:721:2:
> >> include/linux/bits.h:35:4: error: call to 'GENMASK_INPUT_CHECK_FAIL' declared with attribute error: Invalid bit numbers  
...
>    include/linux/mm.h:4641:27: note: in expansion of macro 'GENMASK'
>     #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \

This might be a real bug.

The bit of mm.h is:

#define PP_DMA_INDEX_SHIFT (1 + __fls(PP_SIGNATURE - POISON_POINTER_DELTA))
#if POISON_POINTER_DELTA > 0
/* PP_SIGNATURE includes POISON_POINTER_DELTA, so limit the size of the DMA
 * index to not overlap with that if set
 */
#define PP_DMA_INDEX_BITS MIN(32, __ffs(POISON_POINTER_DELTA) - PP_DMA_INDEX_SHIFT)
#else
/* Use the lowest bit of PAGE_OFFSET if there's at least 8 bits available; see above */
#define PP_DMA_INDEX_MIN_OFFSET (1 << (PP_DMA_INDEX_SHIFT + 8))
#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && \
			    PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
			    !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
			      MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)

#endif

#define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \
				  PP_DMA_INDEX_SHIFT)

I've no idea what the values are, but the 'hi' bit number must exceed that
of 'long'.
The __ffs() probably stop it being an 'integer constant expression'
making it just a 'compile time constant' - which I added a test for.

	David