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Subject: Re: [PATCH] mm/hotplug: Adjust shrink_zone_span() to keep the old
 logic
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, richardw.yang@linux.intel.com,
        mhocko@suse.com, osalvador@suse.de
References: <20200206053912.1211-1-bhe@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <f2b6b83d-8a96-2aef-f132-f66d7009df9c@redhat.com>
Date:   Thu, 6 Feb 2020 10:48:02 +0100
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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@redhat.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.

-- 
Thanks,

David / dhildenb