[PATCH v3 1/2] mm/memcg: alignment memcg_data define condition

[alexs]

From: "Alex Shi (Tencent)" <alexs@xxxxxxxxxx>

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. And selected SLAB_OBJ_EXT for MEMCG, just for
SLAB_MATCH(memcg_data, obj_exts), even no other relationship between them.

Above action make memcg_data exposed and include SLAB_OBJ_EXT for
!MEMCG. That's incorrect in logcial and pay on code size.

As Vlastimil Babka suggested, let's add _unused_slab_obj_ext for
SLAB_MATCH for slab.obj_exts while !MEMCG. That could resolve the match
issue, clean up the feature logical. And decouple the SLAB_OBJ_EXT from
MEMCG in next patch.

Signed-off-by: Alex Shi (Tencent) <alexs@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Yoann Congal <yoann.congal@xxxxxxxx>
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
---
v1->v3: take Vlastimil's suggestion and move SLAB_OBJ_EXT/MEMCG decouple
to 2nd patch.
---
 include/linux/mm_types.h | 8 ++++++--
 mm/slab.h                | 4 ++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ef09c4eef6d3..4ac3abc673d3 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -180,8 +180,10 @@ struct page {
 	/* 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_ext;
 #endif
 
 	/*
@@ -343,8 +345,10 @@ struct folio {
 			};
 			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_ext;
 #endif
 #if defined(WANT_PAGE_VIRTUAL)
 			void *virtual;
diff --git a/mm/slab.h b/mm/slab.h
index 3586e6183224..8ffdd4f315f8 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -98,7 +98,11 @@ 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);
+#else
+SLAB_MATCH(_unused_slab_obj_ext, obj_exts);
+#endif
 #endif
 #undef SLAB_MATCH
 static_assert(sizeof(struct slab) <= sizeof(struct page));
-- 
2.43.0