linux-kernel - Re: [PATCH] mm, page_alloc: fix build_zonerefs

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Message-ID: <Yk7TMKBAkuSVZRLT@dhcp22.suse.cz>
Date:   Thu, 7 Apr 2022 14:04:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm, page_alloc: fix build_zonerefs_node()

On Thu 07-04-22 13:58:44, David Hildenbrand wrote:
[...]
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3589febc6d31..130a2feceddc 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6112,10 +6112,8 @@ static int build_zonerefs_node(pg_data_t *pgdat, struct zoneref *zonerefs)
> >  	do {
> >  		zone_type--;
> >  		zone = pgdat->node_zones + zone_type;
> > -		if (managed_zone(zone)) {
> > -			zoneref_set_zone(zone, &zonerefs[nr_zones++]);
> > -			check_highest_zone(zone_type);
> > -		}
> > +		zoneref_set_zone(zone, &zonerefs[nr_zones++]);
> > +		check_highest_zone(zone_type);
> >  	} while (zone_type);
> >  
> >  	return nr_zones;
> 
> I don't think having !populated zones in the zonelist is a particularly
> good idea. Populated vs !populated changes only during page
> onlininge/offlining.
> 
> If I'm not wrong, with your patch we'd even include ZONE_DEVICE here ...

What kind of problem that would cause? The allocator wouldn't see any
pages at all so it would fallback to the next one. Maybe kswapd would
need some tweak to have a bail out condition but as mentioned in the
thread already. !populated or !managed for that matter are not all that
much different from completely depleted zones. The fact that we are
making that distinction has led to some bugs and I suspect it makes the
code more complex without a very good reason.

> I'd vote for going with the simple fix first, which should be good
> enough AFAIKT.

yes, see the other reply

-- 
Michal Hocko
SUSE Labs