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>
diff --git a/mm/slab.c b/mm/slab.c
index 878354b..9d4c7b5 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -157,6 +157,17 @@
#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
@@ -2016,6 +2027,10 @@
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
@@ -2258,6 +2273,12 @@
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);