mm/memcg: alignment memcg_data define condition

commit 21c690a349ba ("mm: introduce slabobj_ext to support slab object
extensions") changed the folio/page->memcg_data define condition from
MEMCG to SLAB_OBJ_EXT. This action make memcg_data exposed while !MEMCG.

As Vlastimil Babka suggested, let's add _unused_slab_obj_exts for
SLAB_MATCH for slab.obj_exts while !MEMCG. That could resolve the match
issue, clean up the feature logical.

Signed-off-by: Alex Shi (Tencent) <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Yoann Congal <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Vlastimil Babka <[e-mail]>

include/linux/mm_types.h

···
 	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
 	atomic_t _refcount;
 
-#ifdef CONFIG_SLAB_OBJ_EXT
+#ifdef CONFIG_MEMCG
 	unsigned long memcg_data;
+#elif defined(CONFIG_SLAB_OBJ_EXT)
+	unsigned long _unused_slab_obj_exts;
 #endif
 
 	/*
···
  * @_hugetlb_cgroup_rsvd: Do not use directly, use accessor in hugetlb_cgroup.h.
  * @_hugetlb_hwpoison: Do not use directly, call raw_hwp_list_head().
  * @_deferred_list: Folios to be split under memory pressure.
+ * @_unused_slab_obj_exts: Placeholder to match obj_exts in struct slab.
  *
  * A folio is a physically, virtually and logically contiguous set
  * of bytes.  It is a power-of-two in size, and it is aligned to that
···
 			};
 			atomic_t _mapcount;
 			atomic_t _refcount;
-#ifdef CONFIG_SLAB_OBJ_EXT
+#ifdef CONFIG_MEMCG
 			unsigned long memcg_data;
+#elif defined(CONFIG_SLAB_OBJ_EXT)
+			unsigned long _unused_slab_obj_exts;
 #endif
 #if defined(WANT_PAGE_VIRTUAL)
 			void *virtual;

mm/slab.h

···
 SLAB_MATCH(flags, __page_flags);
 SLAB_MATCH(compound_head, slab_cache);	/* Ensure bit 0 is clear */
 SLAB_MATCH(_refcount, __page_refcount);
-#ifdef CONFIG_SLAB_OBJ_EXT
+#ifdef CONFIG_MEMCG
 SLAB_MATCH(memcg_data, obj_exts);
+#elif defined(CONFIG_SLAB_OBJ_EXT)
+SLAB_MATCH(_unused_slab_obj_exts, obj_exts);
 #endif
 #undef SLAB_MATCH
 static_assert(sizeof(struct slab) <= sizeof(struct page));