linux-kernel - Re: [PATCH v2 2/5] mm: zswap: calculate limits only when updated

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Date: Fri, 5 Apr 2024 11:26:19 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Nhat Pham <nphamcs@...il.com>,
	Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] mm: zswap: calculate limits only when updated

On Fri, Apr 05, 2024 at 05:35:07AM +0000, Yosry Ahmed wrote:
> Currently, we calculate the zswap global limit, and potentially the
> acceptance threshold in the zswap, in pages in the zswap store path.
> This is unnecessary because the values rarely change.
> 
> Instead, precalculate the them when the module parameters are updated,
> which should be rare. Since we are adding custom handlers for setting
> the percentages now, add proper validation that they are <= 100.
> 
> Suggested-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Nice! Getting that stuff out of the hotpath!

Two comments below:

> @@ -684,6 +703,43 @@ static int zswap_enabled_param_set(const char *val,
>  	return ret;
>  }
>  
> +static int __zswap_percent_param_set(const char *val,
> +				     const struct kernel_param *kp)
> +{
> +	unsigned int n;
> +	int ret;
> +
> +	ret = kstrtouint(val, 10, &n);
> +	if (ret || n > 100)
> +		return -EINVAL;
> +
> +	return param_set_uint(val, kp);
> +}
> +
> +static int zswap_max_pool_param_set(const char *val,
> +				    const struct kernel_param *kp)
> +{
> +	int err = __zswap_percent_param_set(val, kp);
> +
> +	if (!err) {
> +		zswap_update_max_pages();
> +		zswap_update_accept_thr_pages();
> +	}
> +
> +	return err;
> +}
> +
> +static int zswap_accept_thr_param_set(const char *val,
> +				      const struct kernel_param *kp)
> +{
> +	int err = __zswap_percent_param_set(val, kp);
> +
> +	if (!err)
> +		zswap_update_accept_thr_pages();
> +
> +	return err;
> +}

I think you can keep this simple and just always update both if
anything changes for whatever reason. It's an extremely rare event
after all. That should cut it from 3 functions to 1.

Note that totalram_pages can also change during memory onlining and
offlining. For that you need a memory notifier that also calls that
refresh function. It's simple enough, though, check out the code
around register_memory_notifier() in drivers/xen/balloon.c.