Re: [PATCH] mm: Fix protection usage propagation

[Roman Gushchin <guro@fb.com>]

On Mon, Aug 03, 2020 at 05:32:31PM +0200, Michal Hocko wrote:
> From: Michal Koutný <mkoutny@suse.com>
> 
> When workload runs in cgroups that aren't directly below root cgroup and
> their parent specifies reclaim protection, it may end up ineffective.
> 
> The reason is that propagate_protected_usage() is not called in all
> hierarchy up. All the protected usage is incorrectly accumulated in the
> workload's parent. This means that siblings_low_usage is overestimated
> and effective protection underestimated. Even though it is transitional
> phenomenon (uncharge path does correct propagation and fixes the wrong
> children_low_usage), it can undermine the indended protection
> unexpectedly.

Indeed, good catch!

> 
> The fix is simply updating children_low_usage in respective ancestors
> also in the charging path.
> 
> Fixes: 230671533d64 ("mm: memory.low hierarchical behavior")
> Cc: stable # 4.18+
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

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

Thank you!

> ---
> 
> Hi,
> I am sending this patch on behalf of Michal Koutny who is currently
> on vacation and didn't get to post it before he left.
> 
> We have noticed this problem while seeing a swap out in a descendant of
> a protected memcg (intermediate node) while the parent was conveniently
> under its protection limit and the memory pressure was external
> to that hierarchy. Michal has pinpointed this down to the wrong
> siblings_low_usage which led to the unwanted reclaim.
> 
> I am adding my ack directly in this submission.
> 
>  mm/page_counter.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index c56db2d5e159..b4663844c9b3 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -72,7 +72,7 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
>  		long new;
>  
>  		new = atomic_long_add_return(nr_pages, &c->usage);
> -		propagate_protected_usage(counter, new);
> +		propagate_protected_usage(c, new);
>  		/*
>  		 * This is indeed racy, but we can live with some
>  		 * inaccuracy in the watermark.
> @@ -116,7 +116,7 @@ bool page_counter_try_charge(struct page_counter *counter,
>  		new = atomic_long_add_return(nr_pages, &c->usage);
>  		if (new > c->max) {
>  			atomic_long_sub(nr_pages, &c->usage);
> -			propagate_protected_usage(counter, new);
> +			propagate_protected_usage(c, new);
>  			/*
>  			 * This is racy, but we can live with some
>  			 * inaccuracy in the failcnt.
> @@ -125,7 +125,7 @@ bool page_counter_try_charge(struct page_counter *counter,
>  			*fail = c;
>  			goto failed;
>  		}
> -		propagate_protected_usage(counter, new);
> +		propagate_protected_usage(c, new);
>  		/*
>  		 * Just like with failcnt, we can live with some
>  		 * inaccuracy in the watermark.
> -- 
> 2.27.0
>