[akpm-mm:mm-new 222/223] include/linux/damon.h:869 damon_rand() warn: always true condition '(span <= (~0)) => (0-u32max <= u32max)'

[akpm-mm:mm-new 222/223] include/linux/damon.h:869 damon_rand() warn: always true condition '(span <= (~0)) => (0-u32max <= u32max)'

[kernel test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-new
head:   7505bb13a9bb1f214310915ccc06643119fdafc9
commit: 8b7c38eb9621c71d288eca45c8dd4dc92357dc07 [222/223] mm/damon: replace damon_rand() with a per-ctx lockless PRNG
config: riscv-randconfig-r071-20260509 (https://download.01.org/0day-ci/archive/20260509/202605092036.9mdDJbEZ-lkp@intel.com/config)
compiler: riscv32-linux-gcc (GCC) 8.5.0
smatch: v0.5.0-9065-ge9cc34fd

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/202605092036.9mdDJbEZ-lkp@intel.com/

smatch warnings:
include/linux/damon.h:869 damon_rand() warn: always true condition '(span <= (~0)) => (0-u32max <= u32max)'

vim +869 include/linux/damon.h

   861	
   862	/* Get a random number in [@l, @r) using @ctx's lockless PRNG. */
   863	static inline unsigned long damon_rand(struct damon_ctx *ctx,
   864					       unsigned long l, unsigned long r)
   865	{
   866		unsigned long span = r - l;
   867		u64 rnd;
   868	
 > 869		if (span <= U32_MAX) {
   870			rnd = prandom_u32_state(&ctx->rnd_state);
   871			return l + (unsigned long)((rnd * span) >> 32);
   872		}
   873		rnd = ((u64)prandom_u32_state(&ctx->rnd_state) << 32) |
   874		      prandom_u32_state(&ctx->rnd_state);
   875		return l + mul_u64_u64_shr(rnd, span, 64);
   876	}
   877	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [akpm-mm:mm-new 222/223] include/linux/damon.h:869 damon_rand() warn: always true condition '(span <= (~0)) => (0-u32max <= u32max)'

[SeongJae Park]

On Sat, 09 May 2026 20:44:33 +0800 kernel test robot <lkp@intel.com> wrote:

> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-new
> head:   7505bb13a9bb1f214310915ccc06643119fdafc9
> commit: 8b7c38eb9621c71d288eca45c8dd4dc92357dc07 [222/223] mm/damon: replace damon_rand() with a per-ctx lockless PRNG
> config: riscv-randconfig-r071-20260509 (https://download.01.org/0day-ci/archive/20260509/202605092036.9mdDJbEZ-lkp@intel.com/config)
> compiler: riscv32-linux-gcc (GCC) 8.5.0
> smatch: v0.5.0-9065-ge9cc34fd
> 
> 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/202605092036.9mdDJbEZ-lkp@intel.com/
> 
> smatch warnings:
> include/linux/damon.h:869 damon_rand() warn: always true condition '(span <= (~0)) => (0-u32max <= u32max)'
> 
> vim +869 include/linux/damon.h
> 
>    861	
>    862	/* Get a random number in [@l, @r) using @ctx's lockless PRNG. */
>    863	static inline unsigned long damon_rand(struct damon_ctx *ctx,
>    864					       unsigned long l, unsigned long r)
>    865	{
>    866		unsigned long span = r - l;
>    867		u64 rnd;
>    868	
>  > 869		if (span <= U32_MAX) {
>    870			rnd = prandom_u32_state(&ctx->rnd_state);
>    871			return l + (unsigned long)((rnd * span) >> 32);
>    872		}
>    873		rnd = ((u64)prandom_u32_state(&ctx->rnd_state) << 32) |
>    874		      prandom_u32_state(&ctx->rnd_state);
>    875		return l + mul_u64_u64_shr(rnd, span, 64);
>    876	}
>    877	

The check is for 64bit systems.  I think this warning is ok to be ignored.
Please let me know if I'm missing something.


Thanks,
SJ

[...]