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Date: Thu, 6 Feb 2020 07:08:26 +0800
From: Baoquan He <bhe@redhat.com>
To: Wei Yang <richardw.yang@linux.intel.com>
Subject: Re: [PATCH v6 08/10] mm/memory_hotplug: Don't check for "all holes"
 in shrink_zone_span()
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Cc: linux-s390@vger.kernel.org, x86@kernel.org, linux-ia64@vger.kernel.org,
 Pavel Tatashin <pasha.tatashin@soleen.com>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 02/06/20 at 06:56am, Wei Yang wrote:
> On Wed, Feb 05, 2020 at 10:48:11PM +0800, Baoquan He wrote:
> >Hi Wei Yang,
> >
> >On 02/05/20 at 05:59pm, Wei Yang wrote:
> >> >diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >> >index f294918f7211..8dafa1ba8d9f 100644
> >> >--- a/mm/memory_hotplug.c
> >> >+++ b/mm/memory_hotplug.c
> >> >@@ -393,6 +393,9 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >> > 		if (pfn) {
> >> > 			zone->zone_start_pfn = pfn;
> >> > 			zone->spanned_pages = zone_end_pfn - pfn;
> >> >+		} else {
> >> >+			zone->zone_start_pfn = 0;
> >> >+			zone->spanned_pages = 0;
> >> > 		}
> >> > 	} else if (zone_end_pfn == end_pfn) {
> >> > 		/*
> >> >@@ -405,34 +408,11 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >> > 					       start_pfn);
> >> > 		if (pfn)
> >> > 			zone->spanned_pages = pfn - zone_start_pfn + 1;
> >> >+		else {
> >> >+			zone->zone_start_pfn = 0;
> >> >+			zone->spanned_pages = 0;
> >> >+		}
> >> > 	}
> >> 
> >> If it is me, I would like to take out these two similar logic out.
> >
> >I also like this style. 
> >> 
> >> For example:
> >> 
> >> 	if () {
> >> 	} else if () {
> >> 	} else {
> >> 		goto out;
> >Here the last else is unnecessary, right?
> >
> 
> I am afraid not.
> 
> If the range is not the first or last, we would leave pfn not initialized.

Ah, you are right. I forgot that one. Then pfn can be assigned the
zone_start_pfn as the old code. Then the following logic is the same
as the original code, find_smallest_section_pfn()/find_biggest_section_pfn() 
have done the iteration the old for loop was doing.

	unsigned long pfn = zone_start_pfn;	
	if () {
	} else if () {
	} 

	/* The zone has no valid section */
	if (!pfn) {
        	zone->zone_start_pfn = 0;
        	zone->spanned_pages = 0;
	}