blk-iocost: Avoid using clamp() on inuse in __propagate_weights()

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().

Fixes: 7caa471 ("blkcg: implement blk-iocost")
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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: nathanchance

block/blk-iocost.c

@@ -1098,7 +1098,14 @@ static void __propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse,
 		inuse = DIV64_U64_ROUND_UP(active * iocg->child_inuse_sum,
 					   iocg->child_active_sum);
 	} else {
-		inuse = clamp_t(u32, inuse, 1, active);
+		/*
+		 * It may be tempting to turn this into a clamp expression with
+		 * a lower limit of 1 but active may be 0, which cannot be used
+		 * as an upper limit in that situation. This expression allows
+		 * active to clamp inuse unless it is 0, in which case inuse
+		 * becomes 1.
+		 */
+		inuse = min(inuse, active) ?: 1;
 	}
 
 	iocg->last_inuse = iocg->inuse;