Re: [PATCH 0/6] mm: make movable onlining suck less

[Michal Hocko]

On Wed 05-04-17 12:32:49, Reza Arbab wrote:
> On Wed, Apr 05, 2017 at 05:42:59PM +0200, Michal Hocko wrote:
> >But one thing that is really bugging me is how could you see low pfns in
> >the previous oops. Please drop the last patch and sprinkle printks down
> >the remove_memory path to see where this all go south. I believe that
> >there is something in the initialization code lurking in my code. Please
> >also scratch the pfn_valid check in online_pages diff. It will not help
> >here.
> 
> Got it.
> 
> shrink_pgdat_span: start_pfn=0x10000, end_pfn=0x10100, pgdat_start_pfn=0x0, pgdat_end_pfn=0x20000
> 
> The problem is that pgdat_start_pfn here should be 0x10000. As you
> suspected, it never got set. This fixes things for me.
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 623507f..37c1b63 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -884,7 +884,7 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
> {
> 	unsigned long old_end_pfn = pgdat_end_pfn(pgdat);
> 
> -	if (start_pfn < pgdat->node_start_pfn)
> +	if (!pgdat->node_spanned_pages || start_pfn < pgdat->node_start_pfn)
> 		pgdat->node_start_pfn = start_pfn;

Dang! You are absolutely right. This explains the issue during the
remove_memory. I still fail to see how this makes any difference for the
sysfs file registration though. If anything the pgdat will be larger and
so try_offline_node would check also unrelated node0 but the code will
handle that and eventually offline the node1 anyway. /me still confused.
 
> 	pgdat->node_spanned_pages = max(start_pfn + nr_pages, old_end_pfn) - pgdat->node_start_pfn;
> ---
> 
> Along these lines, maybe we should also do
> 
> -	if (start_pfn < zone->zone_start_pfn)
> +	if (zone_is_empty(zone) || start_pfn < zone->zone_start_pfn)

yes we should.

Thanks a lot!
-- 
Michal Hocko
SUSE Labs