Re: [PATCH] blk-iocost: Avoid using clamp() on inuse in __propagate_weights()

[Tejun Heo <tj@kernel.org>]

On Thu, Dec 12, 2024 at 10:13:29AM -0700, Nathan Chancellor wrote:
> After a recent change to clamp() and its variants [1] that increases the
> coverage of the check that high is greater than low because it can be
> done through inlining, certain build configurations (such as s390
> defconfig) fail to build with clang with:
> 
>   block/blk-iocost.c:1101:11: error: call to '__compiletime_assert_557' declared with 'error' attribute: clamp() low limit 1 greater than high limit active
>    1101 |                 inuse = clamp_t(u32, inuse, 1, active);
>         |                         ^
>   include/linux/minmax.h:218:36: note: expanded from macro 'clamp_t'
>     218 | #define clamp_t(type, val, lo, hi) __careful_clamp(type, val, lo, hi)
>         |                                    ^
>   include/linux/minmax.h:195:2: note: expanded from macro '__careful_clamp'
>     195 |         __clamp_once(type, val, lo, hi, __UNIQUE_ID(v_), __UNIQUE_ID(l_), __UNIQUE_ID(h_))
>         |         ^
>   include/linux/minmax.h:188:2: note: expanded from macro '__clamp_once'
>     188 |         BUILD_BUG_ON_MSG(statically_true(ulo > uhi),                            \
>         |         ^
> 
> __propagate_weights() is called with an active value of zero in
> ioc_check_iocgs(), which results in the high value being less than the
> low value, which is undefined because the value returned depends on the
> order of the comparisons.
> 
> The purpose of this expression is to ensure inuse is not more than
> active and at least 1. This could be written more simply with a ternary
> expression that uses min(inuse, active) as the condition so that the
> value of that condition can be used if it is not zero and one if it is.
> Do this conversion to resolve the error and add a comment to deter
> people from turning this back into clamp().
> 
> Link: https://lore.kernel.org/r/34d53778977747f19cce2abb287bb3e6@AcuMS.aculab.com/ [1]
> Suggested-by: David Laight <david.laight@aculab.com>
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Closes: https://lore.kernel.org/llvm/CA+G9fYsD7mw13wredcZn0L-KBA3yeoVSTuxnss-AEWMN3ha0cA@mail.gmail.com/
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202412120322.3GfVe3vF-lkp@intel.com/
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Acked-by: Tejun Heo <tj@kernel.org>

This likely deserves:

Fixes: 7caa47151ab2 ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+

Thanks.

-- 
tejun