slab: reset slab->obj_ext when freeing and it is OBJEXTS_ALLOC_FAIL

If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
But we do not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
MEMCG_DATA_OBJEXTS currently share the same bit position, during the
release of the associated folio, a VM_BUG_ON_FOLIO() check in
folio_memcg_kmem() is triggered because the OBJEXTS_ALLOC_FAIL flag was
not cleared, causing it to be interpreted as a kmem folio (non-slab)
with MEMCG_OBJEXTS_DATA flag set, which is invalid because
MEMCG_OBJEXTS_DATA is supposed to be set only on slabs.

Another problem that predates sharing the OBJEXTS_ALLOC_FAIL and
MEMCG_DATA_OBJEXTS bits is that on configurations with
is_check_pages_enabled(), the non-cleared bit in page->memcg_data will
trigger a free_page_is_bad() failure "page still charged to cgroup"

When freeing a slab, we clear slab->obj_exts if the obj_ext array has
been successfully allocated. So let's clear it also when the allocation
has failed.

Fixes: 09c4656 ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
Fixes: 7612833 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL")
Link: https://lore.kernel.org/all/20251015141642.700170-1-hao.ge@linux.dev/
Cc: <stable@vger.kernel.org>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: GeHao01994

mm/slub.c

@@ -2170,8 +2170,15 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	struct slabobj_ext *obj_exts;
 
 	obj_exts = slab_obj_exts(slab);
-	if (!obj_exts)
+	if (!obj_exts) {
+		/*
+		 * If obj_exts allocation failed, slab->obj_exts is set to
+		 * OBJEXTS_ALLOC_FAIL. In this case, we end up here and should
+		 * clear the flag.
+		 */
+		slab->obj_exts = 0;
 		return;
+	}
 
 	/*
 	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its