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Date:   Thu, 6 Feb 2020 18:00:29 +0800
From:   Baoquan He <bhe@redhat.com>
To:     David Hildenbrand <david@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
Subject: Re: [PATCH] mm/hotplug: Adjust shrink_zone_span() to keep the old
 logic
Message-ID: <20200206100029.GP8965@MiWiFi-R3L-srv>
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 <20200206093530.GO8965@MiWiFi-R3L-srv>
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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@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.

AFAIK, the page 0 is reserved. No valid zone can start at 0, only empty
zone is. Please correct me if I am wrong.