mm: rework memcg kernel stack accounting

If CONFIG_VMAP_STACK is set, kernel stacks are allocated using
__vmalloc_node_range() with __GFP_ACCOUNT.  So kernel stack pages are
charged against corresponding memory cgroups on allocation and uncharged
on releasing them.

The problem is that we do cache kernel stacks in small per-cpu caches and
do reuse them for new tasks, which can belong to different memory cgroups.

Each stack page still holds a reference to the original cgroup, so the
cgroup can't be released until the vmap area is released.

To make this happen we need more than two subsequent exits without forks
in between on the current cpu, which makes it very unlikely to happen.  As
a result, I saw a significant number of dying cgroups (in theory, up to 2
* number_of_cpu + number_of_tasks), which can't be released even by
significant memory pressure.

As a cgroup structure can take a significant amount of memory (first of
all, per-cpu data like memcg statistics), it leads to a noticeable waste
of memory.

Link: http://lkml.kernel.org/r/20180827162621.30187-1-guro@fb.com
Fixes: ac496bf48d97 ("fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y")
Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 49 insertions, 6 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index f0b58479534f..3c719fec46c5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -223,9 +223,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 		return s->addr;
 	}
 
+	/*
+	 * Allocated stacks are cached and later reused by new threads,
+	 * so memcg accounting is performed manually on assigning/releasing
+	 * stacks to tasks. Drop __GFP_ACCOUNT.
+	 */
 	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
 				     VMALLOC_START, VMALLOC_END,
-				     THREADINFO_GFP,
+				     THREADINFO_GFP & ~__GFP_ACCOUNT,
 				     PAGE_KERNEL,
 				     0, node, __builtin_return_address(0));
 
@@ -248,9 +253,19 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 static inline void free_thread_stack(struct task_struct *tsk)
 {
 #ifdef CONFIG_VMAP_STACK
-	if (task_stack_vm_area(tsk)) {
+	struct vm_struct *vm = task_stack_vm_area(tsk);
+
+	if (vm) {
 		int i;
 
+		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
+			mod_memcg_page_state(vm->pages[i],
+					     MEMCG_KERNEL_STACK_KB,
+					     -(int)(PAGE_SIZE / 1024));
+
+			memcg_kmem_uncharge(vm->pages[i], 0);
+		}
+
 		for (i = 0; i < NR_CACHED_STACKS; i++) {
 			if (this_cpu_cmpxchg(cached_stacks[i],
 					NULL, tsk->stack_vm_area) != NULL)
@@ -351,10 +366,6 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 					    NR_KERNEL_STACK_KB,
 					    PAGE_SIZE / 1024 * account);
 		}
-
-		/* All stack pages belong to the same memcg. */
-		mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
-				     account * (THREAD_SIZE / 1024));
 	} else {
 		/*
 		 * All stack pages are in the same zone and belong to the
@@ -370,6 +381,35 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 	}
 }
 
+static int memcg_charge_kernel_stack(struct task_struct *tsk)
+{
+#ifdef CONFIG_VMAP_STACK
+	struct vm_struct *vm = task_stack_vm_area(tsk);
+	int ret;
+
+	if (vm) {
+		int i;
+
+		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
+			/*
+			 * If memcg_kmem_charge() fails, page->mem_cgroup
+			 * pointer is NULL, and both memcg_kmem_uncharge()
+			 * and mod_memcg_page_state() in free_thread_stack()
+			 * will ignore this page. So it's safe.
+			 */
+			ret = memcg_kmem_charge(vm->pages[i], GFP_KERNEL, 0);
+			if (ret)
+				return ret;
+
+			mod_memcg_page_state(vm->pages[i],
+					     MEMCG_KERNEL_STACK_KB,
+					     PAGE_SIZE / 1024);
+		}
+	}
+#endif
+	return 0;
+}
+
 static void release_task_stack(struct task_struct *tsk)
 {
 	if (WARN_ON(tsk->state != TASK_DEAD))
@@ -807,6 +847,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	if (!stack)
 		goto free_tsk;
 
+	if (memcg_charge_kernel_stack(tsk))
+		goto free_stack;
+
 	stack_vm_area = task_stack_vm_area(tsk);
 
 	err = arch_dup_task_struct(tsk, orig);