From 57e420c84f9ab55ba4c5e2ae9c5f6c8e1ea834d2 Mon Sep 17 00:00:00 2001 From: Nathan Chancellor Date: Thu, 12 Dec 2024 10:13:29 -0700 Subject: [PATCH] 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: 7caa47151ab2 ("blkcg: implement blk-iocost") Link: https://lore.kernel.org/r/34d53778977747f19cce2abb287bb3e6@AcuMS.aculab.com/ [1] Suggested-by: David Laight Reported-by: Linux Kernel Functional Testing Closes: https://lore.kernel.org/llvm/CA+G9fYsD7mw13wredcZn0L-KBA3yeoVSTuxnss-AEWMN3ha0cA@mail.gmail.com/ Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202412120322.3GfVe3vF-lkp@intel.com/ Signed-off-by: Nathan Chancellor Acked-by: Tejun Heo Signed-off-by: Jens Axboe --- block/blk-iocost.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/block/blk-iocost.c b/block/blk-iocost.c index 384aa15e8260..a5894ec9696e 100644 --- a/block/blk-iocost.c +++ b/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;