linux-kernel - Re: [PATCH 2/2] x86,mem-hotplug: modify PGD entry when removing memory

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Date:	Tue, 24 Jun 2014 09:12:33 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, tangchen@...fujitsu.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	guz.fnst@...fujitsu.com, zhangyanfei@...fujitsu.com
Subject: Re: [PATCH 2/2] x86,mem-hotplug: modify PGD entry when removing
 memory

On Tue, 2014-06-24 at 09:31 +0900, Yasuaki Ishimatsu wrote:
> (2014/06/21 3:30), Toshi Kani wrote:
> > On Wed, 2014-06-18 at 15:38 +0900, Yasuaki Ishimatsu wrote:
> >   :
> >> @@ -186,7 +186,12 @@ void sync_global_pgds(unsigned long start, unsigned long end)
> >>   		const pgd_t *pgd_ref = pgd_offset_k(address);
> >>   		struct page *page;
> >>
> >> -		if (pgd_none(*pgd_ref))
> >> +		/*
> >> +		 * When it is called after memory hot remove, pgd_none()
> >> +		 * returns true. In this case (removed == 1), we must clear
> >> +		 * the PGD entries in the local PGD level page.
> >> +		 */
> >> +		if (pgd_none(*pgd_ref) && !removed)
> >>   			continue;
> >>
> >>   		spin_lock(&pgd_lock);
> >> @@ -199,12 +204,18 @@ void sync_global_pgds(unsigned long start, unsigned long end)
> >>   			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
> >>   			spin_lock(pgt_lock);
> >>
> >> -			if (pgd_none(*pgd))
> >> -				set_pgd(pgd, *pgd_ref);
> >> -			else
> 
> >> +			if (!pgd_none(*pgd_ref) && !pgd_none(*pgd))
> >>   				BUG_ON(pgd_page_vaddr(*pgd)
> >>   				       != pgd_page_vaddr(*pgd_ref));
> >>
> >> +			if (removed) {
> >
> > Shouldn't this condition be "else if"?
> 
> The first if sentence checks whether PGDs hit to BUG_ON. And the second
> if sentence checks whether the function was called after hot-removing memory.
> I think that the first if sentence and the second if sentence check different
> things. So I think the condition should be "if" sentence.

When the 1st if sentence is true, you have no additional operation and
the 2nd if sentence is redundant. But I agree that the two ifs can be
logically separated. So:

Acked-by: Toshi Kani <toshi.kani@...com>

Thanks,
-Toshi

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