Re: [PATCH] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself

[Roman Gushchin <guro@fb.com>]

On Fri, Dec 27, 2019 at 07:43:53AM -0500, Yafang Shao wrote:
> memory.{emin, elow} are set in mem_cgroup_protected(), and the values of
> them won't be changed until next recalculation in this function. After
> either or both of them are set, the next reclaimer to relcaim this memcg
> may be a different reclaimer, e.g. this memcg is also the root memcg of
> the new reclaimer, and then in mem_cgroup_protection() in get_scan_count()
> the old values of them will be used to calculate scan count, that is not
> proper. We should reset them to zero in this case.
> 
> Here's an example of this issue.
> 
>     root_mem_cgroup
>          /
>         A   memory.max=1024M memory.min=512M memory.current=800M
> 
> Once kswapd is waked up, it will try to scan all MEMCGs, including
> this A, and it will assign memory.emin of A with 512M.
> After that, A may reach its hard limit(memory.max), and then it will
> do memcg reclaim. Because A is the root of this reclaimer, so it will
> not calculate its memory.emin. So the memory.emin is the old value
> 512M, and then this old value will be used in
> mem_cgroup_protection() in get_scan_count() to get the scan count.
> That is not proper.
> 
> Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Chris Down <chris@chrisdown.name>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: stable@vger.kernel.org
> ---
>  mm/memcontrol.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 601405b..bb3925d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6287,8 +6287,17 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>  
>  	if (!root)
>  		root = root_mem_cgroup;
> -	if (memcg == root)
> +	if (memcg == root) {
> +		/*
> +		 * Reset memory.(emin, elow) for reclaiming the memcg
> +		 * itself.
> +		 */
> +		if (memcg != root_mem_cgroup) {
> +			memcg->memory.emin = 0;
> +			memcg->memory.elow = 0;
> +		}

I'm sorry, that didn't bring it from scratch, but I doubt that zeroing effecting
protection is correct. Imagine a simple config: a large cgroup subtree with memory.max
set on the top level. Reaching this limit doesn't mean that all protection
configuration inside the tree can be ignored.

Instead we should respect memory.low/max set by a user on this level
(look at the parent == root case), maybe clamped by memory.high/max.

Thanks!