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Date:   Wed, 19 Apr 2023 13:03:11 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Haifeng Xu <haifeng.xu@shopee.com>
Cc:     mhocko@suse.com, hannes@cmpxchg.org, roman.gushchin@linux.dev,
        shakeelb@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm, vmpressure: remove redundant check in vmpressure()
Message-Id: <20230419130311.587b7273eca2512c07b32bd0@linux-foundation.org>
In-Reply-To: <20230419092007.186938-1-haifeng.xu@shopee.com>
References: <20230419092007.186938-1-haifeng.xu@shopee.com>
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On Wed, 19 Apr 2023 09:20:07 +0000 Haifeng Xu <haifeng.xu@shopee.com> wrote:

> There are three places, vmpressure_prio(), shrink_node_memcgs() and
> shrink_node(), which invoke vmpressure(). But only shrink_node_memcgs()
> sets tree to false and the memcg used in it is not NULL, so we don't
> check it again in vmpressure().
> 
> ...
>
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -284,7 +284,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>  		enum vmpressure_levels level;
>  
>  		/* For now, no users for root-level efficiency */
> -		if (!memcg || mem_cgroup_is_root(memcg))
> +		if (mem_cgroup_is_root(memcg))
>  			return;
>  
>  		spin_lock(&vmpr->sr_lock);

try_to_free_pages()->
  do_try_to_free_pages()->
    vmpressure_prio()-> 
      vmpressure()->
        crash

what am I missing here?


It does appear that vmpressure() could be simplified with

	if (!memcg)
		memcg = root_mem_cgroup;

so the test you identified goes away and the memcg_to_vmpressure() call
becomes simpler.  But that's such a small change it doesn't seem worth
the effort.