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

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Date:   Wed, 5 Apr 2017 08:42:39 +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 Tue 04-04-17 16:43:39, Reza Arbab wrote:
> On Tue, Apr 04, 2017 at 09:41:22PM +0200, Michal Hocko wrote:
> >On Tue 04-04-17 13:30:13, Reza Arbab wrote:
> >>I think I found another edge case.  You
> >>get an oops when removing all of a node's memory:
> >>
> >>__nr_to_section
> >>__pfn_to_section
> >>find_biggest_section_pfn
> >>shrink_pgdat_span
> >>__remove_zone
> >>__remove_section
> >>__remove_pages
> >>arch_remove_memory
> >>remove_memory
> >
> >Is this something new or an old issue? I believe the state after the
> >online should be the same as before. So if you onlined the full node
> >then there shouldn't be any difference. Let me have a look...
> 
> It's new. Without this patchset, I can repeatedly
> add_memory()->online_movable->offline->remove_memory() all of a node's
> memory.

This is quite unexpected because the code obviously cannot handle the
first memory section. Could you paste /proc/zoneinfo and
grep . -r /sys/devices/system/memory/auto_online_blocks/memory*, after
onlining for both patched and unpatched kernels?

> >From 1b08ecef3e8ebcef585fe8f2b23155be54cce335 Mon Sep 17 00:00:00 2001
> >From: Michal Hocko <mhocko@...e.com>
> >Date: Tue, 4 Apr 2017 21:09:00 +0200
> >Subject: [PATCH] mm, hotplug: get rid of zone/node shrinking
> >
> ...%<...
> >---
> >mm/memory_hotplug.c | 207 ----------------------------------------------------
> >1 file changed, 207 deletions(-)
> 
> Okay, getting further. With this I can again repeatedly add and remove, but
> now I'm seeing a weird variation of that earlier issue:
> 
> 1. add_memory(), online_movable
>   /sys/devices/system/node/nodeX/memoryY symlinks are created.
> 
> 2. offline, remove_memory()
>   The node is offlined, since all memory has been removed, so all of
>   /sys/devices/system/node/nodeX is gone. This is normal.
> 
> 3. add_memory(), online_movable
>   The node is onlined, so /sys/devices/system/node/nodeX is recreated,
>   and the memory is added, but just like earlier in this email thread,
>   the memoryY links are not there.

Could you add some printks to see why the sysfs creation failed please?
-- 
Michal Hocko
SUSE Labs