fork: optimize memcg_charge_kernel_stack() a bit

Since commit f1c1a9ee ("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.

[akpm@linux-foundation.org: remove assertion that PAGE_SIZE is a multiple of 1k]
Link: https://lkml.kernel.org/r/20230508064458.32855-1-haifeng.xu@shopee.comSigned-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/fork.c

@@ -252,23 +252,19 @@ 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);
 	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;
 }