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Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Pavel Tatashin <pasha.tatashin@oracle.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, gregkh@linuxfoundation.org,
        mingo@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        dan.j.williams@intel.com, jack@suse.cz, mawilcox@microsoft.com,
        jglisse@redhat.com, Souptick Joarder <jrdr.linux@gmail.com>,
        kirill.shutemov@linux.intel.com, Vlastimil Babka <vbabka@suse.cz>,
        osalvador@techadventures.net, yasu.isimatu@gmail.com,
        malat@debian.org, Mel Gorman <mgorman@suse.de>,
        iamjoonsoo.kim@lge.com
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <0be90c23-e5a0-2628-c671-9923d8e45b0a@redhat.com>
Date:   Mon, 30 Jul 2018 17:03:27 +0200
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On 30.07.2018 16:50, Michal Hocko wrote:
> On Mon 30-07-18 16:42:27, David Hildenbrand wrote:
>> On 30.07.2018 16:10, Michal Hocko wrote:
>>> On Mon 30-07-18 15:51:45, David Hildenbrand wrote:
>>>> On 30.07.2018 15:30, Pavel Tatashin wrote:
>>> [...]
>>>>> Hi David,
>>>>>
>>>>> Have you figured out why we access struct pages during hot-unplug for
>>>>> offlined memory? Also, a panic trace would be useful in the patch.
>>>>
>>>> __remove_pages() needs a zone as of now (e.g. to recalculate if the zone
>>>> is contiguous). This zone is taken from the first page of memory to be
>>>> removed. If the struct pages are uninitialized that value is random and
>>>> we might even get an invalid zone.
>>>>
>>>> The zone is also used to locate pgdat.
>>>>
>>>> No stack trace available so far, I'm just reading the code and try to
>>>> understand how this whole memory hotplug/unplug machinery works.
>>>
>>> Yes this is a mess (evolution of the code called otherwise ;) [1].
>>
>> So I guess I should not feel bad if I am having problems understanding
>> all the details? ;)
>>
>>> Functionality has been just added on top of not very well thought
>>> through bases. This is a nice example of it. We are trying to get a zone
>>> to 1) special case zone_device 2) recalculate zone state. The first
>>> shouldn't be really needed because we should simply rely on altmap.
>>> Whether it is used for zone device or not. 2) shouldn't be really needed
>>> if the section is offline and we can check that trivially.
>>>
>>
>> About 2, I am not sure if this is the case and that easy. To me it looks
>> more like remove_pages() fixes up things that should be done in
>> offline_pages(). Especially, if the same memory was onlined/offlined to
>> different zones we might be in trouble (looking at code on a very high
>> level view).
> 
> Well, this might be possible. Hotplug remove path was on my todo list
> for a long time. I didn't get that far TBH. shrink_zone_span begs for
> some attention.
> 

So i guess we agree that the right fix for this is to not touch struct
pages when removing memory, correct?

-- 

Thanks,

David / dhildenb