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

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Date:   Wed, 5 Apr 2017 21:39:34 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>
Cc:     Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Tang Chen <tangchen@...fujitsu.com>, qiuxishi@...wei.com,
        Kani Toshimitsu <toshi.kani@....com>, slaoub@...il.com,
        Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Zhang Zhen <zhenzhang.zhang@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dan Williams <dan.j.williams@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Lai Jiangshan <laijs@...fujitsu.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 0/6] mm: make movable onlining suck less

On Wed 05-04-17 20:15:02, Michal Hocko wrote:
> 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.

OK, so I've managed to convince my kvm setup to create a node without
any memory initially but I cannot seem to be able to reach
node_set_offline in try_offline_node because check_and_unmap_cpu_on_node
fails for me even when I offline cpus bound to the node because we are
using for_each_present_cpu in check_cpu_on_node so I would have to start
with a cpuless node or find a way how to hotremove those cpus.
-- 
Michal Hocko
SUSE Labs