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Date:   Thu, 31 Mar 2022 17:33:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Jonathan Corbet <corbet@lwn.net>, Yu Zhao <yuzhao@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
Message-Id: <20220331173350.1fe09370479a4a6f916b477d@linux-foundation.org>
In-Reply-To: <20220331084151.2600229-1-yosryahmed@google.com>
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On Thu, 31 Mar 2022 08:41:51 +0000 Yosry Ahmed <yosryahmed@google.com> wrote:

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6355,6 +6355,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
>  	return nbytes;
>  }
>  
> +static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> +			      size_t nbytes, loff_t off)
> +{
> +	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> +	unsigned int nr_retries = MAX_RECLAIM_RETRIES;
> +	unsigned long nr_to_reclaim, nr_reclaimed = 0;
> +	int err;
> +
> +	buf = strstrip(buf);
> +	err = page_counter_memparse(buf, "", &nr_to_reclaim);
> +	if (err)
> +		return err;
> +
> +	while (nr_reclaimed < nr_to_reclaim) {
> +		unsigned long reclaimed;
> +
> +		if (signal_pending(current))
> +			break;
> +
> +		reclaimed = try_to_free_mem_cgroup_pages(memcg,
> +						nr_to_reclaim - nr_reclaimed,
> +						GFP_KERNEL, true);
> +
> +		if (!reclaimed && !nr_retries--)
> +			break;
> +
> +		nr_reclaimed += reclaimed;
> +	}

Is there any way in which this can be provoked into triggering the
softlockup detector?

Is it optimal to do the MAX_RECLAIM_RETRIES loop in the kernel? 
Would additional flexibility be gained by letting userspace handle
retrying?