linux-kernel - Re: [PATCH v3] mm: memcontrol: fix kernel stack account

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Date:   Wed, 3 Mar 2021 10:01:37 -0800
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <akpm@...ux-foundation.org>, <shakeelb@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v3] mm: memcontrol: fix kernel stack account

On Wed, Mar 03, 2021 at 11:18:43PM +0800, Muchun Song wrote:
> 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. But this is not a real world
> problem, just a problem found by reading the code. So there is no actual
> data to showing how much impact it has on users.
> 
> 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.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Michal Hocko <mhocko@...e.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Roman Gushchin <guro@...com>

Thanks!

> ---
> Changelog in v3:
>  - Remove BUG_ON().
>  - Update commit log.
> 
> Changelog in v2:
>  - Rework commit log suggested by Michal.
> 
>  Thanks to Michal and Shakeel for review.
> 
>  kernel/fork.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d66cd1014211..242fdad6972b 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -379,14 +379,17 @@ 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
> +		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
>