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

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Date:	Wed, 25 Jun 2014 08:29:04 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Toshi Kani <toshi.kani@...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

(2014/06/25 0:12), Toshi Kani wrote:
> 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>

Thank you for your review.

Thanks,
Yasuaki Ishimatsu

>
> Thanks,
> -Toshi
>


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