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

[David Hildenbrand]

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