linux-kernel - Re: [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug

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Date:   Mon, 17 Sep 2018 09:32:27 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Rashmica <rashmica.g@...il.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-acpi@...r.kernel.org,
        xen-devel@...ts.xenproject.org, devel@...uxdriverproject.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Michael Neuling <mikey@...ling.org>,
        Balbir Singh <bsingharora@...il.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Mathieu Malaterre <malat@...ian.org>
Subject: Re: [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages
 called w.o. mem_hotplug_lock

Am 03.09.18 um 02:36 schrieb Rashmica:
> Hi David,
> 
> 
> On 21/08/18 20:44, David Hildenbrand wrote:
> 
>> There seem to be some problems as result of 30467e0b3be ("mm, hotplug:
>> fix concurrent memory hot-add deadlock"), which tried to fix a possible
>> lock inversion reported and discussed in [1] due to the two locks
>> 	a) device_lock()
>> 	b) mem_hotplug_lock
>>
>> While add_memory() first takes b), followed by a) during
>> bus_probe_device(), onlining of memory from user space first took b),
>> followed by a), exposing a possible deadlock.
> 
> Do you mean "onlining of memory from user space first took a),
> followed by b)"? 

Very right, thanks.

> 
>> In [1], and it was decided to not make use of device_hotplug_lock, but
>> rather to enforce a locking order.
>>
>> The problems I spotted related to this:
>>
>> 1. Memory block device attributes: While .state first calls
>>    mem_hotplug_begin() and the calls device_online() - which takes
>>    device_lock() - .online does no longer call mem_hotplug_begin(), so
>>    effectively calls online_pages() without mem_hotplug_lock.
>>
>> 2. device_online() should be called under device_hotplug_lock, however
>>    onlining memory during add_memory() does not take care of that.
>>
>> In addition, I think there is also something wrong about the locking in
>>
>> 3. arch/powerpc/platforms/powernv/memtrace.c calls offline_pages()
>>    without locks. This was introduced after 30467e0b3be. And skimming over
>>    the code, I assume it could need some more care in regards to locking
>>    (e.g. device_online() called without device_hotplug_lock - but I'll
>>    not touch that for now).
> 
> Can you mention that you fixed this in later patches?

Sure!

> 
> 
> The series looks good to me. Feel free to add my reviewed-by:
> 
> Reviewed-by: Rashmica Gupta <rashmica.g@...il.com>
> 

Thanks, r-b only for this patch or all of the series?

-- 

Thanks,

David / dhildenb