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Subject: Re: [PATCH 3/3] mm: Fix missing mem cgroup soft limit tree updates
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.cz>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Ying Huang <ying.huang@intel.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
References: <cover.1612902157.git.tim.c.chen@linux.intel.com>
 <3b6e4e9aa8b3ee1466269baf23ed82d90a8f791c.1612902157.git.tim.c.chen@linux.intel.com>
 <YCMLEB/2IscnaGGh@cmpxchg.org>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <3445ebcd-bc69-ec6e-8995-c95753b5c4a7@linux.intel.com>
Date:   Tue, 9 Feb 2021 14:34:07 -0800
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On 2/9/21 2:22 PM, Johannes Weiner wrote:
> Hello Tim,
> 
> On Tue, Feb 09, 2021 at 12:29:47PM -0800, Tim Chen wrote:
>> @@ -6849,7 +6850,9 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>>  	 * exclusive access to the page.
>>  	 */
>>  
>> -	if (ug->memcg != page_memcg(page)) {
>> +	if (ug->memcg != page_memcg(page) ||
>> +	    /* uncharge batch update soft limit tree on a node basis */
>> +	    (ug->dummy_page && ug->nid != page_to_nid(page))) {
> 
> The fix makes sense to me.
> 
> However, unconditionally breaking up the batch by node can
> unnecessarily regress workloads in cgroups that do not have a soft
> limit configured, and cgroup2 which doesn't have soft limits at
> all. Consider an interleaving allocation policy for example.
> 
> Can you please further gate on memcg->soft_limit != PAGE_COUNTER_MAX,
> or at least on !cgroup_subsys_on_dfl(memory_cgrp_subsys)?
> 

Sure.  Will fix this.

Tim