[PATCH 4/4] kernel/fork: add option to use virtually mapped stacks as fallback

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Virtually mapped stack have two bonuses: it eats order-0 pages and
adds guard page at the end. But it slightly slower if system have
plenty free high-order pages.

This patch adds option to use virtually bapped stack as fallback for
atomic allocation of traditional high-order page.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 arch/Kconfig  |   14 ++++++++++++++
 kernel/fork.c |   11 +++++++++++
 2 files changed, 25 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 400b9e1b2f27..c181ab263e7f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -904,6 +904,20 @@ config VMAP_STACK
 	  the stack to map directly to the KASAN shadow map using a formula
 	  that is incorrect if the stack is in vmalloc space.
 
+config VMAP_STACK_AS_FALLBACK
+	default n
+	bool "Use a virtually-mapped stack as fallback for directly-mapped"
+	depends on VMAP_STACK
+	help
+	  With this option kernel first tries to allocate directly-mapped stack
+	  without calling direct memory reclaim and fallback to vmap stack.
+
+	  Allocation of directly mapped stack faster than vmap if system a lot
+	  of free memory and much slower if all memory is used or fragmented.
+
+	  This option neutralize stack overflow protection but allows to
+	  achieve best performance for syscalls fork() and clone().
+
 config ARCH_OPTIONAL_KERNEL_RWX
 	def_bool n
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 457c9151f3c8..cc61a083954d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -207,6 +207,17 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 	struct vm_struct *stack;
 	int i;
 
+#ifdef CONFIG_VMAP_STACK_AS_FALLBACK
+	struct page *page;
+
+	page = alloc_pages_node(node, THREADINFO_GFP & ~__GFP_DIRECT_RECLAIM,
+				THREAD_SIZE_ORDER);
+	if (page) {
+		tsk->stack_vm_area = NULL;
+		return page_address(page);
+	}
+#endif
+
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
 		stack = this_cpu_xchg(cached_stacks[i], NULL);
 		if (!stack)