[PATCH] fork: optimize memcg_charge_kernel_stack() a bit

[Haifeng Xu]

Since commit f1c1a9ee00e4 ("fork: Move memcg_charge_kernel_stack()
into CONFIG_VMAP_STACK"), memcg_charge_kernel_stack() has been moved
into CONFIG_VMAP_STACK block, so the CONFIG_VMAP_STACK check can be
removed.

Furthermore, memcg_charge_kernel_stack() is only invoked by
alloc_thread_stack_node() instead of dup_task_struct(). If
memcg_kmem_charge_page() fails, the uncharge process is handled in
memcg_charge_kernel_stack() itself instead of free_thread_stack(),
so remove the incorrect comments.

If memcg_charge_kernel_stack() fails to charge pages used by kernel
stack, only charged pages need to be uncharged. It's unnecessary to
uncharge those pages which memory cgroup pointer is NULL.

Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
---
 kernel/fork.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index ea332319dffe..50762a41aa4e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -250,23 +250,20 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm)
 {
 	int i;
 	int ret;
+	int nr_charged = 0;
 
-	BUILD_BUG_ON(IS_ENABLED(CONFIG_VMAP_STACK) && PAGE_SIZE % 1024 != 0);
+	BUILD_BUG_ON(PAGE_SIZE % 1024 != 0);
 	BUG_ON(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
 
 	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
 		ret = memcg_kmem_charge_page(vm->pages[i], GFP_KERNEL, 0);
 		if (ret)
 			goto err;
+		nr_charged++;
 	}
 	return 0;
 err:
-	/*
-	 * If memcg_kmem_charge_page() fails, page's memory cgroup pointer is
-	 * NULL, and memcg_kmem_uncharge_page() in free_thread_stack() will
-	 * ignore this page.
-	 */
-	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+	for (i = 0; i < nr_charged; i++)
 		memcg_kmem_uncharge_page(vm->pages[i], 0);
 	return ret;
 }
-- 
2.25.1