[PATCH v2] mm: memcontrol: fix kernel stack account

[Muchun Song]

For simplification 991e7673859e ("mm: memcontrol: account kernel stack
per node") has changed the per zone vmalloc backed stack pages
accounting to per node. By doing that we have lost a certain precision
because those pages might live in different NUMA nodes. In the end
NR_KERNEL_STACK_KB exported to the userspace might be over estimated on
some nodes while underestimated on others.

This doesn't impose any real problem to correctnes of the kernel
behavior as the counter is not used for any internal processing but it
can cause some confusion to the userspace.

Address the problem by accounting each vmalloc backing page to its own
node.

Fixes: 991e7673859e ("mm: memcontrol: account kernel stack per node")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
Changelog in v2:
 - Rework commit log suggested by Michal.

 Thanks to Michal and Shakeel for review.

 kernel/fork.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d66cd1014211..6e2201feb524 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -379,14 +379,19 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 	void *stack = task_stack_page(tsk);
 	struct vm_struct *vm = task_stack_vm_area(tsk);
 
+	if (vm) {
+		int i;
 
-	/* All stack pages are in the same node. */
-	if (vm)
-		mod_lruvec_page_state(vm->pages[0], NR_KERNEL_STACK_KB,
-				      account * (THREAD_SIZE / 1024));
-	else
+		BUG_ON(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
+
+		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+			mod_lruvec_page_state(vm->pages[i], NR_KERNEL_STACK_KB,
+					      account * (PAGE_SIZE / 1024));
+	} else {
+		/* All stack pages are in the same node. */
 		mod_lruvec_kmem_state(stack, NR_KERNEL_STACK_KB,
 				      account * (THREAD_SIZE / 1024));
+	}
 }
 
 static int memcg_charge_kernel_stack(struct task_struct *tsk)
-- 
2.11.0