SLUB: ensure that the number of objects per slab stays low for high orders

Currently SLUB has no provision to deal with too high page orders that may
be specified on the kernel boot line.  If an order higher than 6 (on a 4k
platform) is generated then we will BUG() because slabs get more than 65535
objects.

Add some logic that decreases order for slabs that have too many objects.
This allow booting with slab sizes up to MAX_ORDER.

For example

	slub_min_order=10

will boot with a default slab size of 4M and reduce slab sizes for small
object sizes to lower orders if the number of objects becomes too big.
Large slab sizes like that allow a concentration of objects of the same
slab cache under as few as possible TLB entries and thus potentially
reduces TLB pressure.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -205,6 +205,11 @@ static inline void ClearSlabDebug(struct page *page)
 #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
 #endif
 
+/*
+ * The page->inuse field is 16 bit thus we have this limitation
+ */
+#define MAX_OBJECTS_PER_SLAB 65535
+
 /* Internal SLUB flags */
 #define __OBJECT_POISON 0x80000000	/* Poison object */
 
@@ -1736,8 +1741,17 @@ static inline int slab_order(int size, int min_objects,
 {
 	int order;
 	int rem;
+	int min_order = slub_min_order;
 
-	for (order = max(slub_min_order,
+	/*
+	 * If we would create too many object per slab then reduce
+	 * the slab order even if it goes below slub_min_order.
+	 */
+	while (min_order > 0 &&
+		(PAGE_SIZE << min_order) >= MAX_OBJECTS_PER_SLAB * size)
+			min_order--;
+
+	for (order = max(min_order,
 				fls(min_objects * size - 1) - PAGE_SHIFT);
 			order <= max_order; order++) {
 
@@ -1751,6 +1765,9 @@ static inline int slab_order(int size, int min_objects,
 		if (rem <= slab_size / fract_leftover)
 			break;
 
+		/* If the next size is too high then exit now */
+		if (slab_size * 2 >= MAX_OBJECTS_PER_SLAB * size)
+			break;
 	}
 
 	return order;
@@ -2037,7 +2054,7 @@ static int calculate_sizes(struct kmem_cache *s)
 	 * The page->inuse field is only 16 bit wide! So we cannot have
 	 * more than 64k objects per slab.
 	 */
-	if (!s->objects || s->objects > 65535)
+	if (!s->objects || s->objects > MAX_OBJECTS_PER_SLAB)
 		return 0;
 	return 1;