linux-kernel - Re: [PATCH] mm/hotplug: Adjust shrink_zone

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Date:   Thu, 6 Feb 2020 11:02:53 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, richardw.yang@...ux.intel.com,
        mhocko@...e.com, osalvador@...e.de
Subject: Re: [PATCH] mm/hotplug: Adjust shrink_zone_span() to keep the old
 logic

On 06.02.20 11:00, Baoquan He wrote:
> On 02/06/20 at 10:48am, David Hildenbrand wrote:
>> On 06.02.20 10:35, Baoquan He wrote:
>>> On 02/06/20 at 09:50am, David Hildenbrand wrote:
>>>> On 06.02.20 06:39, Baoquan He wrote:
>>>>> In commit 950b68d9178b ("mm/memory_hotplug: don't check for "all holes"
>>>>> in shrink_zone_span()"), the zone->zone_start_pfn/->spanned_pages
>>>>> resetting is moved into the if()/else if() branches, if the zone becomes
>>>>> empty. However the 2nd resetting code block may cause misunderstanding.
>>>>>
>>>>> So take the resetting codes out of the conditional checking and handling
>>>>> branches just as the old code does, the find_smallest_section_pfn()and
>>>>> find_biggest_section_pfn() searching have done the the same thing as
>>>>> the old for loop did, the logic is kept the same as the old code. This
>>>>> can remove the possible confusion.
>>>>>
>>>>> Signed-off-by: Baoquan He <bhe@...hat.com>
>>>>> ---
>>>>>  mm/memory_hotplug.c | 14 ++++++--------
>>>>>  1 file changed, 6 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>>> index 089b6c826a9e..475d0d68a32c 100644
>>>>> --- a/mm/memory_hotplug.c
>>>>> +++ b/mm/memory_hotplug.c
>>>>> @@ -398,7 +398,7 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
>>>>>  static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
>>>>>  			     unsigned long end_pfn)
>>>>>  {
>>>>> -	unsigned long pfn;
>>>>> +	unsigned long pfn = zone->zone_start_pfn;
>>>>>  	int nid = zone_to_nid(zone);
>>>>>  
>>>>>  	zone_span_writelock(zone);
>>>>> @@ -414,9 +414,6 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
>>>>>  		if (pfn) {
>>>>>  			zone->spanned_pages = zone_end_pfn(zone) - pfn;
>>>>>  			zone->zone_start_pfn = pfn;
>>>>> -		} else {
>>>>> -			zone->zone_start_pfn = 0;
>>>>> -			zone->spanned_pages = 0;
>>>>>  		}
>>>>>  	} else if (zone_end_pfn(zone) == end_pfn) {
>>>>>  		/*
>>>>> @@ -429,10 +426,11 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
>>>>>  					       start_pfn);
>>>>>  		if (pfn)
>>>>>  			zone->spanned_pages = pfn - zone->zone_start_pfn + 1;
>>>>> -		else {
>>>>> -			zone->zone_start_pfn = 0;
>>>>> -			zone->spanned_pages = 0;
>>>>> -		}
>>>>> +	}
>>>>> +
>>>>> +	if (!pfn) {
>>>>> +		zone->zone_start_pfn = 0;
>>>>> +		zone->spanned_pages = 0;
>>>>>  	}
>>>>>  	zone_span_writeunlock(zone);
>>>>>  }
>>>>>
>>>>
>>>> So, what if your zone starts at pfn 0? Unlikely that we can actually
>>>> offline that, but still it is more confusing than the old code IMHO.
>>>> Then I prefer to drop the second else case as discussed instead.
>>>
>>> Hmm, pfn is initialized as zone->zone_start_pfn, does it matter?
>>> The impossible empty zone won't go wrong if it really happen.
>>>
>>
>> If you offline any memory block that belongs to the lowest zone
>> (zone->zone_start_pfn == 0) but does not fall on a boundary (so that you
>> can actually shrink), you would mark the whole zone offline. That's
>> broken unless I am missing something.
> 
> AFAIK, the page 0 is reserved. No valid zone can start at 0, only empty
> zone is. Please correct me if I am wrong.

At least on x86 it indeed is :) So if this holds true for all archs

Acked-by: David Hildenbrand <david@...hat.com>

Thanks!


-- 
Thanks,

David / dhildenb