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Date:   Wed, 29 Apr 2020 12:06:39 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Chris Down <chris@chrisdown.name>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>,
        Yafang Shao <laoar.shao@gmail.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm, memcg: Decouple e{low,min} state mutations from
 protection checks
Message-ID: <20200429100639.GZ28637@dhcp22.suse.cz>
References: <cover.1588092152.git.chris@chrisdown.name>
 <51ffacb736bb02ecc09c828ebe8f4ec7fda68c48.1588092152.git.chris@chrisdown.name>
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On Tue 28-04-20 19:27:00, Chris Down wrote:
> mem_cgroup_protected currently is both used to set effective low and min
> and return a mem_cgroup_protection based on the result. As a user, this
> can be a little unexpected: it appears to be a simple predicate
> function, if not for the big warning in the comment above about the
> order in which it must be executed.
> 
> This change makes it so that we separate the state mutations from the
> actual protection checks, which makes it more obvious where we need to
> be careful mutating internal state, and where we are simply checking and
> don't need to worry about that.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Yafang Shao <laoar.shao@gmail.com>

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

> ---
>  include/linux/memcontrol.h | 48 +++++++++++++++++++++++++++++---------
>  mm/memcontrol.c            | 30 +++++++-----------------
>  mm/vmscan.c                | 17 ++++----------
>  3 files changed, 49 insertions(+), 46 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index d630af1a4e17..88576b1235b0 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -50,12 +50,6 @@ enum memcg_memory_event {
>  	MEMCG_NR_MEMORY_EVENTS,
>  };
>  
> -enum mem_cgroup_protection {
> -	MEMCG_PROT_NONE,
> -	MEMCG_PROT_LOW,
> -	MEMCG_PROT_MIN,
> -};
> -
>  struct mem_cgroup_reclaim_cookie {
>  	pg_data_t *pgdat;
>  	unsigned int generation;
> @@ -357,8 +351,26 @@ static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
>  		   READ_ONCE(memcg->memory.elow));
>  }
>  
> -enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
> -						struct mem_cgroup *memcg);
> +void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> +				     struct mem_cgroup *memcg);
> +
> +static inline bool mem_cgroup_below_low(struct mem_cgroup *memcg)
> +{
> +	if (mem_cgroup_disabled())
> +		return false;
> +
> +	return READ_ONCE(memcg->memory.elow) >=
> +		page_counter_read(&memcg->memory);
> +}
> +
> +static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
> +{
> +	if (mem_cgroup_disabled())
> +		return false;
> +
> +	return READ_ONCE(memcg->memory.emin) >=
> +		page_counter_read(&memcg->memory);
> +}
>  
>  int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
>  			  gfp_t gfp_mask, struct mem_cgroup **memcgp,
> @@ -838,13 +850,27 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
>  static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
>  						  bool in_low_reclaim)
>  {
> +
> +
> +static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> +						   struct mem_cgroup *memcg);
> +{
> +}
> +
> +static inline void mem_cgroup_protection(struct mem_cgroup *memcg,
> +					 bool in_low_reclaim)
> +{
>  	return 0;
>  }
>  
> -static inline enum mem_cgroup_protection mem_cgroup_protected(
> -	struct mem_cgroup *root, struct mem_cgroup *memcg)
> +static inline bool mem_cgroup_below_low(struct mem_cgroup *memcg)
> +{
> +	return false;
> +}
> +
> +static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
>  {
> -	return MEMCG_PROT_NONE;
> +	return false;
>  }
>  
>  static inline int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b0374be44e9e..317dbbaac603 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6368,27 +6368,21 @@ static unsigned long effective_protection(unsigned long usage,
>  }
>  
>  /**
> - * mem_cgroup_protected - check if memory consumption is in the normal range
> + * mem_cgroup_calculate_protection - calculate and cache effective low and min
>   * @root: the top ancestor of the sub-tree being checked
>   * @memcg: the memory cgroup to check
>   *
>   * WARNING: This function is not stateless! It can only be used as part
>   *          of a top-down tree iteration, not for isolated queries.
> - *
> - * Returns one of the following:
> - *   MEMCG_PROT_NONE: cgroup memory is not protected
> - *   MEMCG_PROT_LOW: cgroup memory is protected as long there is
> - *     an unprotected supply of reclaimable memory from other cgroups.
> - *   MEMCG_PROT_MIN: cgroup memory is protected
>   */
> -enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
> -						struct mem_cgroup *memcg)
> +void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> +				     struct mem_cgroup *memcg)
>  {
>  	unsigned long usage, parent_usage;
>  	struct mem_cgroup *parent;
>  
>  	if (mem_cgroup_disabled())
> -		return MEMCG_PROT_NONE;
> +		return;
>  
>  	if (!root)
>  		root = root_mem_cgroup;
> @@ -6403,22 +6397,22 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>  		 */
>  		WRITE_ONCE(memcg->memory.emin, 0);
>  		WRITE_ONCE(memcg->memory.elow, 0);
> -		return MEMCG_PROT_NONE;
> +		return;
>  	}
>  
>  	usage = page_counter_read(&memcg->memory);
>  	if (!usage)
> -		return MEMCG_PROT_NONE;
> +		return;
>  
>  	parent = parent_mem_cgroup(memcg);
>  	/* No parent means a non-hierarchical mode on v1 memcg */
>  	if (!parent)
> -		return MEMCG_PROT_NONE;
> +		return;
>  
>  	if (parent == root) {
>  		memcg->memory.emin = READ_ONCE(memcg->memory.min);
>  		memcg->memory.elow = memcg->memory.low;
> -		goto out;
> +		return;
>  	}
>  
>  	parent_usage = page_counter_read(&parent->memory);
> @@ -6431,14 +6425,6 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>  	WRITE_ONCE(memcg->memory.elow, effective_protection(usage, parent_usage,
>  			memcg->memory.low, READ_ONCE(parent->memory.elow),
>  			atomic_long_read(&parent->memory.children_low_usage)));
> -
> -out:
> -	if (usage <= memcg->memory.emin)
> -		return MEMCG_PROT_MIN;
> -	else if (usage <= memcg->memory.elow)
> -		return MEMCG_PROT_LOW;
> -	else
> -		return MEMCG_PROT_NONE;
>  }
>  
>  /**
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 72ac38eb8c29..e913c4652341 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2645,14 +2645,15 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  		unsigned long reclaimed;
>  		unsigned long scanned;
>  
> -		switch (mem_cgroup_protected(target_memcg, memcg)) {
> -		case MEMCG_PROT_MIN:
> +		mem_cgroup_calculate_protection(target_memcg, memcg);
> +
> +		if (mem_cgroup_below_min(memcg)) {
>  			/*
>  			 * Hard protection.
>  			 * If there is no reclaimable memory, OOM.
>  			 */
>  			continue;
> -		case MEMCG_PROT_LOW:
> +		} else if (mem_cgroup_below_low(memcg)) {
>  			/*
>  			 * Soft protection.
>  			 * Respect the protection only as long as
> @@ -2664,16 +2665,6 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  				continue;
>  			}
>  			memcg_memory_event(memcg, MEMCG_LOW);
> -			break;
> -		case MEMCG_PROT_NONE:
> -			/*
> -			 * All protection thresholds breached. We may
> -			 * still choose to vary the scan pressure
> -			 * applied based on by how much the cgroup in
> -			 * question has exceeded its protection
> -			 * thresholds (see get_scan_count).
> -			 */
> -			break;
>  		}
>  
>  		reclaimed = sc->nr_reclaimed;
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs