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Date:   Sun, 6 Feb 2022 02:17:26 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Wei Yang <richard.weiyang@gmail.com>, akpm@linux-foundation.org,
        mgorman@techsingularity.net, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/page_alloc: add zone to zonelist if populated
Message-ID: <20220206021726.wknhhvkc7fevkpc4@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
References: <20220203020022.3044-1-richard.weiyang@gmail.com>
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On Thu, Feb 03, 2022 at 10:27:11AM +0100, Michal Hocko wrote:
>On Thu 03-02-22 02:00:22, Wei Yang wrote:
>> During memory hotplug, when online/offline a zone, we need to rebuild
>> the zonelist for all nodes. Current behavior would lose a valid zone in
>> zonelist since only pick up managed_zone.
>> 
>> There are two cases for a zone with memory but still !managed.
>> 
>>   * all pages were allocated via memblock
>>   * all pages were taken by ballooning / virtio-mem
>> 
>> This state maybe temporary, since both of them may release some memory.
>> Then it end up with a managed zone not in zonelist.
>> 
>> This is introduced in 'commit 6aa303defb74 ("mm, vmscan: only allocate
>> and reclaim from zones with pages managed by the buddy allocator")'.
>> This patch restore the behavior.
>
>It has been introduced to fix a problem described in the the changelog
>(FADUMP configuration making kswapd hogging a cpu). You are not
>explaining why the original issue is not possible after this change.
>

The first sight is kswapd deals with pgdat->node_zones, which is not affected
by pgdat->node_zonelists.

For the exact detail, I don't figure that out now. Will need some time to take
a look into. For that commit, I only found this link.
http://lkml.kernel.org/r/20160831195104.GB8119@techsingularity.net If there
are some other discussions, it would be helpful.

>I also think that this is more of theoretical issue than anything that
>is a real life concern. It is good to state that in the changelog as
>well.
>
>That being said I am not against the change but the changelog needs more
>explanation before I can ack it.
>
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> CC: Mel Gorman <mgorman@techsingularity.net>
>> CC: David Hildenbrand <david@redhat.com>
>> Fixes: 6aa303defb74 ("mm, vmscan: only allocate and reclaim from zones with pages managed by the buddy allocator")
>
>Fixes tag should be really used only if the referenced commit breaks
>something. I do not really see this to be the case here.
>

Got it.

>Thanks!
>
>> ---
>>  mm/page_alloc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index de15021a2887..b433a57ee76f 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6092,7 +6092,7 @@ static int build_zonerefs_node(pg_data_t *pgdat, struct zoneref *zonerefs)
>>  	do {
>>  		zone_type--;
>>  		zone = pgdat->node_zones + zone_type;
>> -		if (managed_zone(zone)) {
>> +		if (populated_zone(zone)) {
>>  			zoneref_set_zone(zone, &zonerefs[nr_zones++]);
>>  			check_highest_zone(zone_type);
>>  		}
>> -- 
>> 2.33.1
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me