linux-kernel - Re: [PATCH v6 08/10] mm/memory_hotplug: Don't check for "all holes" in shrink_zone

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Date:   Wed, 5 Feb 2020 14:20:52 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richardw.yang@...ux.intel.com>
Subject: Re: [PATCH v6 08/10] mm/memory_hotplug: Don't check for "all holes"
 in shrink_zone_span()

On 05.02.20 13:43, Baoquan He wrote:
> On 02/04/20 at 03:42pm, David Hildenbrand wrote:
>> On 04.02.20 15:25, Baoquan He wrote:
>>> On 10/06/19 at 10:56am, David Hildenbrand wrote:
>>>> If we have holes, the holes will automatically get detected and removed
>>>> once we remove the next bigger/smaller section. The extra checks can
>>>> go.
>>>>
>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>> Cc: Oscar Salvador <osalvador@...e.de>
>>>> Cc: Michal Hocko <mhocko@...e.com>
>>>> Cc: David Hildenbrand <david@...hat.com>
>>>> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
>>>> Cc: Dan Williams <dan.j.williams@...el.com>
>>>> Cc: Wei Yang <richardw.yang@...ux.intel.com>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>>  mm/memory_hotplug.c | 34 +++++++---------------------------
>>>>  1 file changed, 7 insertions(+), 27 deletions(-)
>>>>
>>>> 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;
>>>
>>> Thinking in which case (zone_start_pfn != start_pfn) and it comes here.
>>
>> Could only happen in case the zone_start_pfn would have been "out of the
>> zone already". If you ask me: unlikely :)
> 
> Yeah, I also think it's unlikely to come here.
> 
> The 'if (zone_start_pfn == start_pfn)' checking also covers the case
> (zone_start_pfn == start_pfn && zone_end_pfn == end_pfn). So this
> zone_start_pfn/spanned_pages resetting can be removed to avoid
> confusion.

At least I would find it more confusing without it (or want a comment
explaining why this does not have to be handled and why the !pfn case is
not possible).

Anyhow, that patch is already upstream and I don't consider this high
priority. Thanks :)

-- 
Thanks,

David / dhildenb