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Date:   Fri, 12 May 2023 17:26:18 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Haifeng Xu <haifeng.xu@shopee.com>
Cc:     luto@kernel.org, bigeasy@linutronix.de, tglx@linutronix.de,
        bristot@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fork: optimize memcg_charge_kernel_stack() a bit
Message-Id: <20230512172618.1120ab4c1e6d3026081c20d9@linux-foundation.org>
In-Reply-To: <20230508064458.32855-1-haifeng.xu@shopee.com>
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On Mon,  8 May 2023 06:44:58 +0000 Haifeng Xu <haifeng.xu@shopee.com> wrote:

> 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.
> 
> ...
>
> --- 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);

This check now deserves some sort of award.  I'll remove it.

>  	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;

We don't really need nr_charged - we could just do

	while (--i >= 0)

but nr_charged is straightforward, which is always a good choice.