[PATCH v2] 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>

When alloc_slab_obj_exts() fails and then later succeeds in allocating
a slab extension vector, it calls handle_failed_objexts_alloc() to
mark all objects in the vector as empty. As a result all objects in
this slab (slabA) will have their extensions set to CODETAG_EMPTY.
Later on if this slabA is used to allocate a slabobj_ext vector for
another slab (slabB), we end up with the slabB->obj_exts pointing to a
slabobj_ext vector that itself has a non-NULL slabobj_ext equal to
CODETAG_EMPTY. When slabB gets freed, free_slab_obj_exts() is called
to free slabB->obj_exts vector. free_slab_obj_exts() calls
mark_objexts_empty(slabB->obj_exts) which will generate a warning
because it expects slabobj_ext vectors to have a NULL obj_ext, not
CODETAG_EMPTY.

Modify mark_objexts_empty() to skip the warning and setting the
obj_ext value if it's already set to CODETAG_EMPTY.

Fixes: 09c46563ff6d ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
Cc: <stable@vger.kernel.org>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
v2: Update the commit message and code comments for greater accuracy,
    incorporating Suren's suggestions.
    Thanks for Suren's help.
---
 mm/slub.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index d4367f25b20d..589c596163c4 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2046,7 +2046,11 @@ 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 */
+
+		if (unlikely(is_codetag_empty(&slab_exts[offs].ref)))
+			return;
+
+		/* codetag should be NULL here */
 		WARN_ON(slab_exts[offs].ref.ct);
 		set_codetag_empty(&slab_exts[offs].ref);
 	}
-- 
2.25.1