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

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Dec 12, 2024 at 07:19:23AM -1000, Tejun Heo wrote:
> 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>

Thanks for the quick response!

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

Thanks, I was wondering if I should have provided those. I'll carry them
forward on any future revisions, as I assume Jens can pick those up with
your tag if this is good enough.

> 
> -- 
> tejun
>