linux-kernel - Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove

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Date:   Mon, 12 Apr 2021 18:02:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 resend] mm/memory_hotplug: Make unpopulated zones PCP
 structures unreachable during hot remove

On 12.04.21 17:27, Mel Gorman wrote:
> On Mon, Apr 12, 2021 at 04:12:11PM +0200, David Hildenbrand wrote:
>>> After v1 of the patch, the race was reduced to the point between the
>>> zone watermark check and the rmqueue_pcplist but yes, it still existed.
>>> Closing it completely was either complex or expensive. Setting
>>> zone->pageset = &boot_pageset before the free would shrink the race
>>> further but that still leaves a potential memory ordering issue.
>>>
>>> While fixable, it's either complex, expensive or both so yes, just leaving
>>> the pageset structures in place would be much more straight-forward
>>> assuming the structures were not allocated in the zone that is being
>>> hot-removed. As things stand, I had trouble even testing zone hot-remove
>>> as there was always a few pages left behind and I did not chase down
>>> why.
>>
>> Can you elaborate? I can reliably trigger zone present pages going to 0 by
>> just hotplugging a DIMM, onlining the memory block devices to the MOVABLE
>> zone, followed by offlining the memory block again.
>>
> 
> For the machine I was testing on, I tried offlining all memory within
> a zone on a NUMA machine. Even if I used movable_zone to create a zone
> or numa=fake to create multiple fake nodes and zones, there was always
> either reserved or pinned pages preventing the full zone being removed.

What can happen is that memblock allocations are still placed into the 
MOVABLE zone -- even with "movablenode" IIRC.

Memory hot(un)plug is usually best tested in QEMU via pc-dimm devices.

-- 
Thanks,

David / dhildenb