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

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Message-ID: <alpine.DEB.2.02.1407281610340.8998@chino.kir.corp.google.com>
Date:	Mon, 28 Jul 2014 16:12:20 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Dave Hansen <dave.hansen@...el.com>
cc:	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, wangnan0@...wei.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] memory hotplug: update the variables after memory
 removed

On Mon, 28 Jul 2014, Dave Hansen wrote:

> 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()
> 

I agree, but I'm not sure the suggestion is any better than the patch.  I 
think it would be better to just figure out whether anything needs to be 
updated in the caller and then call a generic function.

So in arch_add_memory(), do

	end_pfn = PFN_UP(start + size);
	if (end_pfn > max_pfn)
		update_end_of_memory_vars(end_pfn);

and in arch_remove_memory(),

	end_pfn = PFN_UP(start);
	if (end_pfn < max_pfn)
		update_end_of_memory_vars(end_pfn);

and then update_end_of_memory_vars() becomes a three-liner.
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