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Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Jan Kara <jack@suse.cz>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@techadventures.net>,
        YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>,
        Mathieu Malaterre <malat@debian.org>,
        Mel Gorman <mgorman@suse.de>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
References: <20180727165454.27292-1-david@redhat.com>
 <20180730113029.GM24267@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <6cc416e7-522c-a67e-2706-f37aadff084f@redhat.com>
Date:   Mon, 30 Jul 2018 13:53:06 +0200
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On 30.07.2018 13:30, Michal Hocko wrote:
> On Fri 27-07-18 18:54:54, David Hildenbrand wrote:
>> Right now, struct pages are inititalized when memory is onlined, not
>> when it is added (since commit d0dc12e86b31 ("mm/memory_hotplug: optimize
>> memory hotplug")).
>>
>> remove_memory() will call arch_remove_memory(). Here, we usually access
>> the struct page to get the zone of the pages.
>>
>> So effectively, we access stale struct pages in case we remove memory that
>> was never onlined. So let's simply inititalize them earlier, when the
>> memory is added. We only have to take care of updating the zone once we
>> know it. We can use a dummy zone for that purpose.
> 
> I have considered something like this when I was reworking memory
> hotplug to not associate struct pages with zone before onlining and I
> considered this to be rather fragile. I would really not like to get
> back to that again if possible.
> 
>> So effectively, all pages will already be initialized and set to
>> reserved after memory was added but before it was onlined (and even the
>> memblock is added). We only inititalize pages once, to not degrade
>> performance.
> 
> To be honest, I would rather see d0dc12e86b31 reverted. It is late in
> the release cycle and if the patch is buggy then it should be reverted
> rather than worked around. I found the optimization not really
> convincing back then and this is still the case TBH.
> 

If I am not wrong, that's already broken in 4.17, no? What about that?

If we don't care about that, then I agree to reverting said commit for
v4.18.

-- 

Thanks,

David / dhildenb