linux-kernel - Re: [PATCH 2/6] mm, memcg: Prevent memory.max load tearing

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Date:   Mon, 16 Mar 2020 15:56:30 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/6] mm, memcg: Prevent memory.max load tearing

On Thu 12-03-20 17:32:56, Chris Down wrote:
> This one is a bit more nuanced because we have memcg_max_mutex, which is
> mostly just used for enforcing invariants, but we still need to
> READ_ONCE since (despite its name) it doesn't really protect memory.max
> access.
> 
> On write (page_counter_set_max() and memory_max_write()) we use xchg(),
> which uses smp_mb(), so that's already fine.
> 
> Signed-off-by: Chris Down <chris@...isdown.name>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Roman Gushchin <guro@...com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: linux-mm@...ck.org
> Cc: cgroups@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: kernel-team@...com

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

> ---
>  mm/memcontrol.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d32d3c0a16d4..aca2964ea494 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1507,7 +1507,7 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
>  
>  	pr_info("memory: usage %llukB, limit %llukB, failcnt %lu\n",
>  		K((u64)page_counter_read(&memcg->memory)),
> -		K((u64)memcg->memory.max), memcg->memory.failcnt);
> +		K((u64)READ_ONCE(memcg->memory.max)), memcg->memory.failcnt);
>  	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>  		pr_info("swap: usage %llukB, limit %llukB, failcnt %lu\n",
>  			K((u64)page_counter_read(&memcg->swap)),
> @@ -1538,7 +1538,7 @@ unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
>  {
>  	unsigned long max;
>  
> -	max = memcg->memory.max;
> +	max = READ_ONCE(memcg->memory.max);
>  	if (mem_cgroup_swappiness(memcg)) {
>  		unsigned long memsw_max;
>  		unsigned long swap_max;
> @@ -3006,7 +3006,7 @@ static int mem_cgroup_resize_max(struct mem_cgroup *memcg,
>  		 * Make sure that the new limit (memsw or memory limit) doesn't
>  		 * break our basic invariant rule memory.max <= memsw.max.
>  		 */
> -		limits_invariant = memsw ? max >= memcg->memory.max :
> +		limits_invariant = memsw ? max >= READ_ONCE(memcg->memory.max) :
>  					   max <= memcg->memsw.max;
>  		if (!limits_invariant) {
>  			mutex_unlock(&memcg_max_mutex);
> @@ -3753,8 +3753,8 @@ static int memcg_stat_show(struct seq_file *m, void *v)
>  	/* Hierarchical information */
>  	memory = memsw = PAGE_COUNTER_MAX;
>  	for (mi = memcg; mi; mi = parent_mem_cgroup(mi)) {
> -		memory = min(memory, mi->memory.max);
> -		memsw = min(memsw, mi->memsw.max);
> +		memory = min(memory, READ_ONCE(mi->memory.max));
> +		memsw = min(memsw, READ_ONCE(mi->memsw.max));
>  	}
>  	seq_printf(m, "hierarchical_memory_limit %llu\n",
>  		   (u64)memory * PAGE_SIZE);
> @@ -4257,7 +4257,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  	*pheadroom = PAGE_COUNTER_MAX;
>  
>  	while ((parent = parent_mem_cgroup(memcg))) {
> -		unsigned long ceiling = min(memcg->memory.max,
> +		unsigned long ceiling = min(READ_ONCE(memcg->memory.max),
>  					    READ_ONCE(memcg->high));
>  		unsigned long used = page_counter_read(&memcg->memory);
>  
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs