[PATCH v2 3/5] slab: restrict the number of objects in a slab

[Joonsoo Kim]

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_MIN_SIZE ensures that the number of objects in a slab is less or
equal to 256 for a slab with 1 page.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/slab.c b/mm/slab.c
index ec197b9..3cee122 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -157,6 +157,10 @@
 #define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
 #endif
 
+/* We use byte sized index to manage the freelist of a slab */
+#define NR_PER_BYTE (1 << BITS_PER_BYTE)
+#define SLAB_MIN_SIZE (PAGE_SIZE >> BITS_PER_BYTE)
+
 /*
  * true if a page was allocated from pfmemalloc reserves for network-based
  * swap
@@ -2016,6 +2020,10 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
 		if (!num)
 			continue;
 
+		/* We can't handler number of objects more than NR_PER_BYTE */
+		if (num > NR_PER_BYTE)
+			break;
+
 		if (flags & CFLGS_OFF_SLAB) {
 			/*
 			 * Max number of objs-per-slab for caches which
@@ -2258,6 +2266,12 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 		flags |= CFLGS_OFF_SLAB;
 
 	size = ALIGN(size, cachep->align);
+	/*
+	 * We want to restrict the number of objects in a slab to be equal or
+	 * less than 256 in order to manage freelist via byte sized indexes.
+	 */
+	if (size < SLAB_MIN_SIZE)
+		size = ALIGN(SLAB_MIN_SIZE, cachep->align);
 
 	left_over = calculate_slab_order(cachep, size, cachep->align, flags);
 
-- 
1.7.9.5

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