slab: Reuse first bit for OBJEXTS_ALLOC_FAIL

Since the combination of valid upper bits in slab->obj_exts with
OBJEXTS_ALLOC_FAIL bit can never happen,
use OBJEXTS_ALLOC_FAIL == (1ull << 0) as a magic sentinel
instead of (1ull << 2) to free up bit 2.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Alexei Starovoitov

include/linux/memcontrol.h

@@ -341,17 +341,23 @@ enum page_memcg_data_flags {
 	__NR_MEMCG_DATA_FLAGS  = (1UL << 2),
 };
 
+#define __OBJEXTS_ALLOC_FAIL	MEMCG_DATA_OBJEXTS
 #define __FIRST_OBJEXT_FLAG	__NR_MEMCG_DATA_FLAGS
 
 #else /* CONFIG_MEMCG */
 
+#define __OBJEXTS_ALLOC_FAIL	(1UL << 0)
 #define __FIRST_OBJEXT_FLAG	(1UL << 0)
 
 #endif /* CONFIG_MEMCG */
 
 enum objext_flags {
-	/* slabobj_ext vector failed to allocate */
-	OBJEXTS_ALLOC_FAIL = __FIRST_OBJEXT_FLAG,
+	/*
+	 * Use bit 0 with zero other bits to signal that slabobj_ext vector
+	 * failed to allocate. The same bit 0 with valid upper bits means
+	 * MEMCG_DATA_OBJEXTS.
+	 */
+	OBJEXTS_ALLOC_FAIL = __OBJEXTS_ALLOC_FAIL,
 	/* the next bit after the last actual flag */
 	__NR_OBJEXTS_FLAGS  = (__FIRST_OBJEXT_FLAG << 1),
 };

mm/slub.c

@@ -2046,7 +2046,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
 	 * objects with no tag reference. Mark all references in this
 	 * vector as empty to avoid warnings later on.
 	 */
-	if (obj_exts & OBJEXTS_ALLOC_FAIL) {
+	if (obj_exts == OBJEXTS_ALLOC_FAIL) {
 		unsigned int i;
 
 		for (i = 0; i < objects; i++)