alloc_tag: fix set_codetag_empty() when !CONFIG_MEM_ALLOC_PROFILING_DEBUG

It was recently noticed that set_codetag_empty() might be used not only to
mark NULL alloctag references as empty to avoid warnings but also to reset
valid tags (in clear_page_tag_ref()).  Since set_codetag_empty() is
defined as NOOP for CONFIG_MEM_ALLOC_PROFILING_DEBUG=n, such use of
set_codetag_empty() leads to subtle bugs.  Fix set_codetag_empty() for
CONFIG_MEM_ALLOC_PROFILING_DEBUG=n to reset the tag reference.

Link: https://lkml.kernel.org/r/20241130001423.1114965-2-surenb@google.com
Fixes: a8fc28d ("alloc_tag: introduce clear_page_tag_ref() helper function")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: David Wang <00107082@163.com>
Closes: https://lore.kernel.org/lkml/20241124074318.399027-1-00107082@163.com/
Cc: David Wang <00107082@163.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Sourav Panda <souravpanda@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: surenbaghdasaryan

include/linux/alloc_tag.h

@@ -63,7 +63,12 @@ static inline void set_codetag_empty(union codetag_ref *ref)
 #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
 
 static inline bool is_codetag_empty(union codetag_ref *ref) { return false; }
-static inline void set_codetag_empty(union codetag_ref *ref) {}
+
+static inline void set_codetag_empty(union codetag_ref *ref)
+{
+	if (ref)
+		ref->ct = NULL;
+}
 
 #endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */