linux-kernel - [PATCH 2/3] fork: make number of cached stacks (vmapped) configurable using Kbuild

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Date:   Sat,  4 Feb 2017 00:32:39 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     Kees Cook <keescook@...omium.org>, Michal Marek <mmarek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Emese Revfy <re.emese@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Andy Lutomirski <luto@...nel.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mateusz Guzik <mguzik@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Hoeun Ryu <hoeun.ryu@...il.com>
Subject: [PATCH 2/3] fork: make number of cached stacks (vmapped) configurable using Kbuild

Introducing NR_VMAP_STACK_CACHE, the number of cached stacks for virtually
mapped kernel stack can be configurable using Kbuild system.
default value is 2.

Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
---
 arch/Kconfig  | 8 ++++++++
 kernel/fork.c | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index d49a8e6..e94c432 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -849,6 +849,14 @@ 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 NR_VMAP_STACK_CACHE
+	int "Number of cached stacks"
+	default "2"
+	depends on VMAP_STACK
+	---help---
+	  This determines how many stacks can be cached for virtually
+	  mapped kernel stacks.
+
 config ARCH_WANT_RELAX_ORDER
 	bool
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 54421a9..50de6cf 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -166,7 +166,7 @@ void __weak arch_release_thread_stack(unsigned long *stack)
  * vmalloc() is a bit slow, and calling vfree() enough times will force a TLB
  * flush.  Try to minimize the number of calls by caching stacks.
  */
-#define NR_CACHED_STACKS 2
+#define NR_CACHED_STACKS CONFIG_NR_VMAP_STACK_CACHE
 
 struct vm_stack_cache {
 	struct vm_struct **vm_stacks;
-- 
2.7.4