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

[Mel Gorman <mgorman@techsingularity.net>]

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.

-- 
Mel Gorman
SUSE Labs