-Wunneeded-internal-declaration in mm/page_alloc.c

[nickdesaulniers]

kernelci is reporting:

defconfig (arm64, clang-14) — PASS, 0 errors, 2 warnings, 0 section mismatches

Warnings:
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
    1 warning generated.
defconfig+CONFIG_EFI=n (riscv, clang-14) — PASS, 0 errors, 2 warnings, 0 section mismatches

Warnings:
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
    1 warning generated.

aspeed_g5_defconfig (arm, clang-14) — PASS, 0 errors, 12 warnings, 0 section mismatches
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
allnoconfig (i386, clang-14) — PASS, 0 errors, 2 warnings, 0 section mismatches
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
allnoconfig (x86_64, clang-14) — PASS, 0 errors, 2 warnings, 0 section mismatches
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
i386_defconfig (i386, clang-14) — PASS, 0 errors, 2 warnings, 0 section mismatches
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]
multi_v7_defconfig (arm, clang-14) — PASS, 0 errors, 15 warnings, 0 section mismatches
    mm/page_alloc.c:131:40: warning: variable 'pagesets' is not needed and will not be emitted [-Wunneeded-internal-declaration]

[nathanchance]

Caused by https://git.kernel.org/next/linux-next/c/9983a9d577db415c41099a20a5637ab25dd3c240.

9983a9d577db415c41099a20a5637ab25dd3c240 is the first bad commit
commit 9983a9d577db415c41099a20a5637ab25dd3c240
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Tue Feb 8 18:08:02 2022 +0100

    locking/local_lock: Make the empty local_lock_*() function a macro.

    It has been said that local_lock() does not add any overhead compared to
    preempt_disable() in a !LOCKDEP configuration. A micro benchmark showed
    an unexpected result which can be reduced to the fact that local_lock()
    was not entirely optimized away.
    In the !LOCKDEP configuration local_lock_acquire() is an empty static
    inline function. On x86 the this_cpu_ptr() argument of that function is
    fully evaluated leading to an additional mov+add instructions which are
    not needed and not used.

    Replace the static inline function with a macro. The typecheck() macro
    ensures that the argument is of proper type while the resulting
    disassembly shows no traces of this_cpu_ptr().

    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Waiman Long <longman@redhat.com>
    Link: https://lkml.kernel.org/r/YgKjciR60fZft2l4@linutronix.de

 include/linux/local_lock_internal.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Prior to that change, the static inline functions would prevent clang from seeing that pagesets is technically unused under CONFIG_PREEMPT_RT=n + CONFIG_DEBUG_LOCK_ALLOC=n since it doesn't do cross function analysis. But now that these functions are macros, clang can see that the variable won't be needed in the final assembly thus the warning. It isn't wrong... I am not really sure of an immediate way to fix this.

[nickdesaulniers]

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7ff1efc84205..597a26f62173 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -128,7 +128,7 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
 struct pagesets {
        local_lock_t lock;
 };
-static DEFINE_PER_CPU(struct pagesets, pagesets) = {
+__maybe_unused static DEFINE_PER_CPU(struct pagesets, pagesets) = {
        .lock = INIT_LOCAL_LOCK(lock),
 };

?

[nathanchance]

Oh duh... I'll send that along shortly with your Suggested-by.

[nickdesaulniers]

submitted: https://lore.kernel.org/r/20220215184322.440969-1-nathan@kernel.org/

[nathanchance]

Tentatively picked up by akpm: https://lore.kernel.org/r/20220215210334.2127EC340EB@smtp.kernel.org/

[nathanchance]

Fixed: https://git.kernel.org/next/linux-next/c/ab7e803d908317d0c44bb491a817db79de773106