linux-kernel - Re: [PATCH v2] mm: memcontrol: fix kernel stack account

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Date:   Wed, 3 Mar 2021 11:25:12 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     guro@...com, hannes@...xchg.org, akpm@...ux-foundation.org,
        shakeelb@...gle.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: memcontrol: fix kernel stack account

On Wed 03-03-21 17:39:56, 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.
> 
> 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.

You have skipped over one part of the changelog I have proposed and that
is to provide an actual data.

> Address the problem by accounting each vmalloc backing page to its own
> node.
> 
> Fixes: 991e7673859e ("mm: memcontrol: account kernel stack per node")

Fixes tag might make somebody assume this is worth backporting but I
highly doubt so.

> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

Anyway
Acked-by: Michal Hocko <mhocko@...e.com>

as the patch is correct with one comment below

> ---
> 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);

I do not think we need this BUG_ON. What kind of purpose does it serve?

> +
> +		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

-- 
Michal Hocko
SUSE Labs