linux-kernel - Re: [PATCH] memory hotplug: update the variables after memory removed

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Message-ID: <53D6685C.1060509@intel.com>
Date:	Mon, 28 Jul 2014 08:12:28 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Zhang Zhen <zhenzhang.zhang@...wei.com>, shaohui.zheng@...el.com,
	mgorman@...e.de, mingo@...hat.com, Linux MM <linux-mm@...ck.org>,
	linux-kernel@...r.kernel.org
CC:	wangnan0@...wei.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] memory hotplug: update the variables after memory removed

On 07/28/2014 05:32 AM, Zhang Zhen wrote:
> -static void  update_end_of_memory_vars(u64 start, u64 size)
> +static void  update_end_of_memory_vars(u64 start, u64 size, bool flag)
>  {
> -	unsigned long end_pfn = PFN_UP(start + size);
> -
> -	if (end_pfn > max_pfn) {
> -		max_pfn = end_pfn;
> -		max_low_pfn = end_pfn;
> -		high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
> +	unsigned long end_pfn;
> +
> +	if (flag) {
> +		end_pfn = PFN_UP(start + size);
> +		if (end_pfn > max_pfn) {
> +			max_pfn = end_pfn;
> +			max_low_pfn = end_pfn;
> +			high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
> +		}
> +	} else {
> +		end_pfn = PFN_UP(start);
> +		if (end_pfn < max_pfn) {
> +			max_pfn = end_pfn;
> +			max_low_pfn = end_pfn;
> +			high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
> +		}
>  	}
>  }

I would really prefer not to see code like this.

This patch takes a small function that did one thing, copies-and-pastes
its code 100%, subtly changes it, and makes it do two things.  The only
thing to tell us what the difference between these two subtly different
things is a variable called 'flag'.  So the variable is useless in
trying to figure out what each version is supposed to do.

But, this fixes a pretty glaring deficiency in the memory remove code.

I would suggest making two functions.  Make it clear that one is to be
used at remove time and the other at add time.  Maybe

	move_end_of_memory_vars_down()
and
	move_end_of_memory_vars_up()

?
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