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 <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9260abdd67df..d015dec02bf3 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -201,6 +201,17 @@ struct kmem_cache {
 #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