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Subject: Re: [PATCH] mm/memory_hotplug: Make unpopulated zones PCP structures
 unreachable during hot remove
To:     Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Minchan Kim <minchan@kernel.org>
References: <20210409120957.GM3697@techsingularity.net>
 <YHBL0e8s+EesIyDl@dhcp22.suse.cz> <YHBNDEAw1OqIWwb5@dhcp22.suse.cz>
 <20210409134221.GO3697@techsingularity.net> <YHBmxwH41WEHuVJj@dhcp22.suse.cz>
 <20210409151259.GP3697@techsingularity.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <9317b74e-f706-27eb-6e5c-3dc0f3ee98b8@redhat.com>
Date:   Fri, 9 Apr 2021 21:05:25 +0200
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>>> zone_pcp_reset still needs to exist to drain the remaining vmstats or
>>> it'll break 5a883813845a ("memory-hotplug: fix zone stat
>>> mismatch").
>>
>> Are you sure we are reseting vmstats in the hotremove. I do not see
>> anything like that. Maybe this was needed at the time. I will double
>> check.
> 
> zone_pcp_reset calls drain_zonestat to apply the per-cpu vmstat deltas
> to the atomic per-zone and global stats.
> 
> If anything, the minimal "fix" is to simply delete IRQ disable/enable on
> the grounds that IRQs protect nothing and assume the existing hotplug
> paths guarantees the PCP cannot be used after zone_pcp_enable(). That

^ that sounds sane to me

> should be the case already because all the pages have been freed and
> there is nothing to even put into the PCPs but I worried that the PCP
> structure itself might still be reachable even if it's useless which is
> why I freed the structure once they could not be reached via zonelists.
> 


-- 
Thanks,

David / dhildenb