linux-kernel - Re: [PATCH v1] mm: inititalize struct pages when adding a section

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Date:   Fri, 27 Jul 2018 20:01:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
        mingo@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        dan.j.williams@...el.com, Michal Hocko <mhocko@...e.com>,
        jack@...e.cz, mawilcox@...rosoft.com, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>,
        kirill.shutemov@...ux.intel.com, Vlastimil Babka <vbabka@...e.cz>,
        osalvador@...hadventures.net, yasu.isimatu@...il.com,
        malat@...ian.org, Mel Gorman <mgorman@...e.de>,
        iamjoonsoo.kim@....com
Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section

On 27.07.2018 19:25, Pavel Tatashin wrote:
> Hi David,
> 
> On Fri, Jul 27, 2018 at 12:55 PM David Hildenbrand <david@...hat.com> 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.
> 
> Yeah, this is a bug, thank you for catching it.
> 
>> 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.
>>
>> 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.
> 
> Yes, but we still add one more npages loop, so there will be some
> performance degradation, but not severe.
> 
> There are many conflicts with linux-next, please sync before sending
> out next patch.

Indeed, although I rebased, I was working on a branch based on Linus
tree ...

[...]

>>  not_early:
>>                 page = pfn_to_page(pfn);
>> -               __init_single_page(page, pfn, zone, nid);
>> -               if (context == MEMMAP_HOTPLUG)
>> -                       SetPageReserved(page);
>> +               if (context == MEMMAP_HOTPLUG) {
>> +                       /* everything but the zone was inititalized */
>> +                       set_page_zone(page, zone);
>> +                       set_page_virtual(page, zone);
>> +               } else
>> +                       init_single_page(page, pfn, zone, nid);
>>
> 
> Please add a new function:
> memmap_init_zone_hotplug() that will handle only the zone and virtual
> fields for onlined hotplug memory.
> 
> Please remove: "enum memmap_context context" from everywhere.

All your comments make sense. Will look into the details next week and
send a new version.

Thanks and enjoy your weekend!


-- 

Thanks,

David / dhildenb