lib/alloc_tag: fix RCU imbalance in pgalloc_tag_get()

put_page_tag_ref() should be called only when get_page_tag_ref() returns a
valid reference because only in that case get_page_tag_ref() enters RCU
read section while put_page_tag_ref() will call rcu_read_unlock() even if
the provided reference is NULL.  Fix pgalloc_tag_get() which does not
follow this rule causing RCU imbalance.  Add a warning in
put_page_tag_ref() to catch any future mistakes.

Link: https://lkml.kernel.org/r/20240601233840.617458-1-surenb@google.com
Fixes: cc92eba1 ("mm: fix non-compound multi-order memory accounting in __free_pages")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202405271029.6d2f9c4c-lkp@intel.comAcked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Kees Cook <keescook@chromium.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pgalloc_tag.h

@@ -37,6 +37,9 @@ static inline union codetag_ref *get_page_tag_ref(struct page *page)
 
 static inline void put_page_tag_ref(union codetag_ref *ref)
 {
+	if (WARN_ON(!ref))
+		return;
+
 	page_ext_put(page_ext_from_codetag_ref(ref));
 }
 
@@ -102,10 +105,12 @@ static inline struct alloc_tag *pgalloc_tag_get(struct page *page)
 		union codetag_ref *ref = get_page_tag_ref(page);
 
 		alloc_tag_sub_check(ref);
-		if (ref && ref->ct)
+		if (ref) {
+			if (ref->ct)
 				tag = ct_to_alloc_tag(ref->ct);
 			put_page_tag_ref(ref);
 		}
+	}
 
 	return tag;
 }