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Subject: Re: [PATCH] mm/hotplug: Adjust shrink_zone_span() to keep the old
 logic
To:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org
Cc:     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>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <7ecaf36f-9f70-05bd-05fc-6dec82b7d559@redhat.com>
Date:   Thu, 6 Feb 2020 09:50:07 +0100
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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.

-- 
Thanks,

David / dhildenb