[PATCH v3] fork: free vmapped stacks in cache when cpus are offline

[Hoeun Ryu <hoeun.ryu@gmail.com>]

 Using virtually mapped stack, kernel stacks are allocated via vmalloc.
In the current implementation, two stacks per cpu can be cached when
tasks are freed and the cached stacks are used again in task duplications.
but the cached stacks may remain unfreed even when cpu are offline.
 By adding a cpu hotplug callback to free the cached stacks when a cpu
goes offline, the pages of the cached stacks are not wasted.

Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
---
Changes in v3:
 fix misuse of per-cpu api
 fix location of function definition within CONFIG_VMAP_STACK

Changes in v2:
 remove cpuhp callback for `startup`, only `teardown` callback is installed.

 kernel/fork.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 937ba59..8fad87ba 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -168,6 +168,24 @@ void __weak arch_release_thread_stack(unsigned long *stack)
  */
 #define NR_CACHED_STACKS 2
 static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
+
+static int free_vm_stack_cache(unsigned int cpu)
+{
+	struct vm_struct **cached_vm_stacks = per_cpu_ptr(cached_stacks, cpu);
+	int i;
+
+	for (i = 0; i < NR_CACHED_STACKS; i++) {
+		struct vm_struct **vm_stack = &cached_vm_stacks[i];
+
+		if (*vm_stack == NULL)
+			continue;
+
+		vfree((*vm_stack)->addr);
+		*vm_stack = NULL;
+	}
+
+	return 0;
+}
 #endif
 
 static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
@@ -456,6 +474,11 @@ void __init fork_init(void)
 	for (i = 0; i < UCOUNT_COUNTS; i++) {
 		init_user_ns.ucount_max[i] = max_threads/2;
 	}
+
+#ifdef CONFIG_VMAP_STACK
+	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "vm_stack_cache",
+			  NULL, free_vm_stack_cache);
+#endif
 }
 
 int __weak arch_dup_task_struct(struct task_struct *dst,
-- 
2.7.4