This is a note to let you know that I've just added the patch titled
blk-iocost: Avoid using clamp() on inuse in __propagate_weights()
to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
blk-iocost-avoid-using-clamp-on-inuse-in-__propagate.patch
and it can be found in the queue-6.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit cbd4b00e15391c5ae95f61b5db20744b74b7cd8f
Author: Nathan Chancellor <nathan@xxxxxxxxxx>
Date: Thu Dec 12 10:13:29 2024 -0700
blk-iocost: Avoid using clamp() on inuse in __propagate_weights()
[ Upstream commit 57e420c84f9ab55ba4c5e2ae9c5f6c8e1ea834d2 ]
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@xxxxxxxxxxxxxxxx/ [1]
Suggested-by: David Laight <david.laight@xxxxxxxxxx>
Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
Closes: https://lore.kernel.org/llvm/CA+G9fYsD7mw13wredcZn0L-KBA3yeoVSTuxnss-AEWMN3ha0cA@xxxxxxxxxxxxxx/
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202412120322.3GfVe3vF-lkp@xxxxxxxxx/
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;
