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

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Date:   Tue, 13 Apr 2021 10:49:20 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     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>,
        David Hildenbrand <david@...hat.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 Tue, Apr 13, 2021 at 11:36:08AM +0200, Vlastimil Babka wrote:
> On 4/12/21 4:08 PM, Mel Gorman wrote:
> > On Mon, Apr 12, 2021 at 02:40:18PM +0200, Vlastimil Babka wrote:
> >> On 4/12/21 2:08 PM, Mel Gorman wrote:
> >
> > 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.
> 
> I would expect this is not possible, at least for ZONE_MOVABLE, as the percpu
> allocations should be GFP_KERNEL.

True.

> And it's not realistic to expect offlining to
> succeed at all without using ZONE_MOVABLE.
> 
> AFAIK even Oscar's work on using the node to self-contain its own structures is
> only applicable to struct pages, not percpu allocations?

That I don't know as I didn't check although in general, it would be
somewhat unfortunate if per-cpu structures were remote. It wouldn't be
critical given that they'll be in cache assuming the per-cpu structures
are not straddling cache lines.

-- 
Mel Gorman
SUSE Labs