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Date: Fri, 25 Jan 2019 11:50:08 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Dave Hansen <dave.hansen@intel.com>
Subject: Re: [RFC PATCH] x86, numa: always initialize all possible nodes
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Cc: Tony Luck <tony.luck@intel.com>, linux-ia64@vger.kernel.org,
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On Thu 24-01-19 11:10:50, Dave Hansen wrote:
> On 1/24/19 6:17 AM, Michal Hocko wrote:
> > and nr_cpus set to 4. The underlying reason is tha the device is bound
> > to node 2 which doesn't have any memory and init_cpu_to_node only
> > initializes memory-less nodes for possible cpus which nr_cpus restrics.
> > This in turn means that proper zonelists are not allocated and the page
> > allocator blows up.
> 
> This looks OK to me.
> 
> Could we add a few DEBUG_VM checks that *look* for these invalid
> zonelists?  Or, would our existing list debugging have caught this?

Currently we simply blow up because those zonelists are NULL. I do not
think we have a way to check whether an existing zonelist is actually 
_correct_ other thatn check it for NULL. But what would we do in the
later case?

> Basically, is this bug also a sign that we need better debugging around
> this?

My earlier patch had a debugging printk to display the zonelists and
that might be worthwhile I guess. Basically something like this

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2e097f336126..c30d59f803fb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5259,6 +5259,11 @@ static void build_zonelists(pg_data_t *pgdat)
 
 	build_zonelists_in_node_order(pgdat, node_order, nr_nodes);
 	build_thisnode_zonelists(pgdat);
+
+	pr_info("node[%d] zonelist: ", pgdat->node_id);
+	for_each_zone_zonelist(zone, z, &pgdat->node_zonelists[ZONELIST_FALLBACK], MAX_NR_ZONES-1)
+		pr_cont("%d:%s ", zone_to_nid(zone), zone->name);
+	pr_cont("\n");
 }
 
 #ifdef CONFIG_HAVE_MEMORYLESS_NODES
-- 
Michal Hocko
SUSE Labs