Re: [PATCH] mm, page_alloc: fix build_zonerefs_node()

From: David Hildenbrand
Date: Thu Apr 07 2022 - 08:12:49 EST

On 07.04.22 14:04, Michal Hocko wrote:
> 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 assume performance problems. Assume you have an ordinary system with
multiple NUMA nodes and no MOVABLE memory. Most nodes will only have
ZONE_NORMAL. Yet, you'd include ZONE_DMA* and ZONE_MOVABLE that will
always remain empty to be traversed on each and every allocation
fallback. Of course, we could measure, but IMHO at least *that* part of
memory onlining/offlining is not the complicated part :D

Populated vs. !populated is under pretty good control via page
onlining/offlining. We have to be careful with "managed pages", because
that's a moving target, especially with memory ballooning. And I assume
that's the bigger source of bugs.

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

I think we were composing almost simultaneously :)


David / dhildenb