slab: restrict the number of objects in a slab

To prepare to implement byte sized index for managing the freelist
of a slab, we should restrict the number of objects in a slab to be less
or equal to 256, since byte only represent 256 different values.
Setting the size of object to value equal or more than newly introduced
SLAB_OBJ_MIN_SIZE ensures that the number of objects in a slab is less or
equal to 256 for a slab with 1 page.

If page size is rather larger than 4096, above assumption would be wrong.
In this case, we would fall back on 2 bytes sized index.

If minimum size of kmalloc is less than 16, we use it as minimum object
size and give up this optimization.

Signed-off-by: Joonsoo Kim <[e-mail]>
Signed-off-by: Pekka Enberg <[e-mail]>

include/linux/slab.h

···
 #ifndef KMALLOC_SHIFT_LOW
 #define KMALLOC_SHIFT_LOW	5
 #endif
+
+/*
+ * This restriction comes from byte sized index implementation.
+ * Page size is normally 2^12 bytes and, in this case, if we want to use
+ * byte sized index which can represent 2^8 entries, the size of the object
+ * should be equal or greater to 2^12 / 2^8 = 2^4 = 16.
+ * If minimum size of kmalloc is less than 16, we use it as minimum object
+ * size and give up to use byte sized index.
+ */
+#define SLAB_OBJ_MIN_SIZE	(KMALLOC_SHIFT_LOW < 4 ? \
+				(1 << KMALLOC_SHIFT_LOW) : 16)
 #endif
 
 #ifdef CONFIG_SLUB

mm/slab.c

···
 #define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
 #endif
 
+#define FREELIST_BYTE_INDEX (((PAGE_SIZE >> BITS_PER_BYTE) \
+				<= SLAB_OBJ_MIN_SIZE) ? 1 : 0)
+
+#if FREELIST_BYTE_INDEX
+typedef unsigned char freelist_idx_t;
+#else
+typedef unsigned short freelist_idx_t;
+#endif
+
+#define SLAB_OBJ_MAX_NUM (1 << sizeof(freelist_idx_t) * BITS_PER_BYTE)
+
 /*
  * true if a page was allocated from pfmemalloc reserves for network-based
  * swap
···
 		if (!num)
 			continue;
 
+		/* Can't handle number of objects more than SLAB_OBJ_MAX_NUM */
+		if (num > SLAB_OBJ_MAX_NUM)
+			break;
+
 		if (flags & CFLGS_OFF_SLAB) {
 			/*
 			 * Max number of objs-per-slab for caches which
···
 		flags |= CFLGS_OFF_SLAB;
 
 	size = ALIGN(size, cachep->align);
+	/*
+	 * We should restrict the number of objects in a slab to implement
+	 * byte sized index. Refer comment on SLAB_OBJ_MIN_SIZE definition.
+	 */
+	if (FREELIST_BYTE_INDEX && size < SLAB_OBJ_MIN_SIZE)
+		size = ALIGN(SLAB_OBJ_MIN_SIZE, cachep->align);
 
 	left_over = calculate_slab_order(cachep, size, cachep->align, flags);