[PATCH] codetag: debug: Handle existing CODETAG_EMPTY in mark_objexts_empty for slabobj_ext

[Hao Ge <hao.ge@linux.dev>]

From: Hao Ge <gehao@kylinos.cn>

Even though obj_exts was created with the __GFP_NO_OBJ_EXT flag,
objects in the same slab may have their extensions allocated via
alloc_slab_obj_exts, and handle_failed_objexts_alloc may be called
within alloc_slab_obj_exts to set their codetag to CODETAG_EMPTY.

Therefore, both NULL and CODETAG_EMPTY are valid for the codetag of
slabobj_ext, as we do not need to re-set it to CODETAG_EMPTY if it
is already CODETAG_EMPTY. It also resolves the warning triggered when
the codetag is CODETAG_EMPTY during slab freeing.

Fixes: 09c46563ff6d ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 mm/slub.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index d4367f25b20d..cda8f75b72e7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2046,7 +2046,17 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 	if (slab_exts) {
 		unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
 						 obj_exts_slab, obj_exts);
-		/* codetag should be NULL */
+
+		/*
+		 * codetag should be either NULL or CODETAG_EMPTY.
+		 * When the same slab calls handle_failed_objexts_alloc,
+		 * it will set us to CODETAG_EMPTY.
+		 *
+		 * If codetag is already CODETAG_EMPTY, no action is needed here.
+		 */
+		if (unlikely(is_codetag_empty(&slab_exts[offs].ref)))
+			return;
+
 		WARN_ON(slab_exts[offs].ref.ct);
 		set_codetag_empty(&slab_exts[offs].ref);
 	}
-- 
2.25.1