[PATCH 1/1] lib/alloc_tag: fix RCU imbalance in pgalloc_tag_get()

[Suren Baghdasaryan <surenb@google.com>]

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.

Fixes: cc92eba1c88b ("mm: fix non-compound multi-order memory accounting in __free_pages")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202405271029.6d2f9c4c-lkp@intel.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
---
 include/linux/pgalloc_tag.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
index 86ba5d33e43b..9cacadbd61f8 100644
--- a/include/linux/pgalloc_tag.h
+++ b/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));
 }
 
@@ -102,9 +105,11 @@ 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)
-			tag = ct_to_alloc_tag(ref->ct);
-		put_page_tag_ref(ref);
+		if (ref) {
+			if (ref->ct)
+				tag = ct_to_alloc_tag(ref->ct);
+			put_page_tag_ref(ref);
+		}
 	}
 
 	return tag;

base-commit: 065d3634d60843b8e338d405b844cc7f2e5e1c66
-- 
2.45.1.288.g0e0cd299f1-goog