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

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Message-ID: <2868343a-745b-e2b6-7e78-d5649c00ee31@redhat.com>
Date:   Wed, 5 Feb 2020 14:38:18 +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 14:34, Baoquan He wrote:
> On 02/05/20 at 02:20pm, David Hildenbrand wrote:
>> 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).
> 
> I don't get why being w/o it will be more confusing, but it's OK since
> it doesn't impact anything. 

Because we could actually BUG_ON(!pfn) here, right? Only having a "if
(pfn)" leaves the reader wondering "why is the other case not handled".

> 
>>
>> Anyhow, that patch is already upstream and I don't consider this high
>> priority. Thanks :)
> 
> Yeah, noticed you told Wei the status in another patch thread, I am fine
> with it, just leave it to you to decide. Thanks.

I am fairly busy right now. Can you send a patch (double-checking and
making this eventually unconditional?). Thanks!

-- 
Thanks,

David / dhildenb