linux-kernel - Re: [PATCH] mm: Set min_free_kbytes with user_min_free_kbytes when user_min_free

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Message-Id: <20210928152919.df87f6c8194316ff1407cdc7@linux-foundation.org>
Date:   Tue, 28 Sep 2021 15:29:19 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Liangcai Fan <liangcaifan19@...il.com>
Cc:     liangcai.fan@...soc.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, zhang.lyra@...il.com
Subject: Re: [PATCH] mm: Set min_free_kbytes with user_min_free_kbytes when
 user_min_free_kbytes is preferred

On Tue, 28 Sep 2021 20:23:17 +0800 Liangcai Fan <liangcaifan19@...il.com> wrote:

> The 'min_free_kbytes' and 'user_min_free_kbytes' maybe inconsistent
> after a few times of memory hotplug.

What does "inconsistent" mean here?

Please describe the problem in more detail, perhaps with examples.

> When 'new_min_free_kbytes' is not larger than 'user_min_free_kbytes',
> set 'min_free_kbytes' with 'user_min_free_kbytes' rather than leave
> it as the 'new_min_free_kbytes' calculated for the last time.
> 
> Signed-off-by: Liangcai Fan <liangcaifan19@...il.com>
> Cc: Chunyan Zhang <zhang.lyra@...il.com>
> ---
>  mm/page_alloc.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b37435c..ddf9dc1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8467,6 +8467,12 @@ int __meminit init_per_zone_wmark_min(void)
>  		if (min_free_kbytes > 262144)
>  			min_free_kbytes = 262144;
>  	} else {
> +		/*
> +		 * Set 'min_free_kbytes' with 'user_min_free_kbytes' rather than
> +		 * leave it as the 'new_min_free_kbytes' calculated for the last
> +		 * time.
> +		 */

This comment explains what the code is doing, which is almost always
obvious from reading the code!  A better comment will describe *why*
the code is doing whatever is does.   "why, not what", please.

> +		min_free_kbytes = user_min_free_kbytes;
>  		pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
>  				new_min_free_kbytes, user_min_free_kbytes);
>  	}