sh: Support kernel stacks smaller than a page.

This follows the powerpc commit f6a616800e
'[POWERPC] Fix kernel stack allocation alignment'.

SH has traditionally forced the thread order to be relative to the page
size, so there were never any situations where the same bug was
triggered by slub. Regardless, the usage of > 8kB stacks for the larger
page sizes is overkill, so we switch to using slab allocations there,
as per the powerpc change.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/mm/init.c

@@ -265,6 +265,35 @@ void free_initrd_mem(unsigned long start, unsigned long end)
 }
 #endif
 
+#if THREAD_SHIFT < PAGE_SHIFT
+static struct kmem_cache *thread_info_cache;
+
+struct thread_info *alloc_thread_info(struct task_struct *tsk)
+{
+	struct thread_info *ti;
+
+	ti = kmem_cache_alloc(thread_info_cache, GFP_KERNEL);
+	if (unlikely(ti == NULL))
+		return NULL;
+#ifdef CONFIG_DEBUG_STACK_USAGE
+	memset(ti, 0, THREAD_SIZE);
+#endif
+	return ti;
+}
+
+void free_thread_info(struct thread_info *ti)
+{
+	kmem_cache_free(thread_info_cache, ti);
+}
+
+void thread_info_cache_init(void)
+{
+	thread_info_cache = kmem_cache_create("thread_info", THREAD_SIZE,
+					      THREAD_SIZE, 0, NULL);
+	BUG_ON(thread_info_cache == NULL);
+}
+#endif /* THREAD_SHIFT < PAGE_SHIFT */
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 int arch_add_memory(int nid, u64 start, u64 size)
 {