mm/ksm: fix -Wsometimes-uninitialized from clang-21 in advisor_mode_show()

After a recent change in clang to expose uninitialized warnings from const
variables [1], there is a false positive warning from the if statement in
advisor_mode_show().

  mm/ksm.c:3687:11: error: variable 'output' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
   3687 |         else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
        |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/ksm.c:3690:33: note: uninitialized use occurs here
   3690 |         return sysfs_emit(buf, "%s\n", output);
        |                                        ^~~~~~

Rewrite the if statement to implicitly make KSM_ADVISOR_NONE the else
branch so that it is obvious to the compiler that ksm_advisor can only be
KSM_ADVISOR_NONE or KSM_ADVISOR_SCAN_TIME due to the assignments in
advisor_mode_store().

Link: https://lkml.kernel.org/r/20250715-ksm-fix-clang-21-uninit-warning-v1-1-f443feb4bfc4@kernel.org
Fixes: 66790e9a735b ("mm/ksm: add sysfs knobs for advisor")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2100
Link: https://github.com/llvm/llvm-project/commit/2464313eef01c5b1edf0eccf57a32cdee01472c7
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/ksm.c b/mm/ksm.c
index 8583fb91ef136ee2f064a45575cd38f06d58772c..a9d3e719e08993eff5d9106ca2cb52bf58cfabd0 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -3669,10 +3669,10 @@ static ssize_t advisor_mode_show(struct kobject *kobj,
 {
        const char *output;
 
-       if (ksm_advisor == KSM_ADVISOR_NONE)
-               output = "[none] scan-time";
-       else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+       if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
                output = "none [scan-time]";
+       else
+               output = "[none] scan-time";
 
        return sysfs_emit(buf, "%s\n", output);
 }