linux-kernel - Re: "mm: use early_pfn_to_nid in page_ext

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Date:   Fri, 7 Jul 2017 14:00:03 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Yang Shi <yang.shi@...aro.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: "mm: use early_pfn_to_nid in page_ext_init" broken on some
 configurations?

On 07/04/2017 07:17 AM, Joonsoo Kim wrote:
>> 
>> Still, backporting b8f1a75d61d8 fixes this:
>> 
>> [    1.538379] allocated 738197504 bytes of page_ext
>> [    1.539340] Node 0, zone      DMA: page owner found early allocated 0 pages
>> [    1.540179] Node 0, zone    DMA32: page owner found early allocated 33 pages
>> [    1.611173] Node 0, zone   Normal: page owner found early allocated 96755 pages
>> [    1.683167] Node 1, zone   Normal: page owner found early allocated 96575 pages
>> 
>> No panic, notice how it allocated more for page_ext, and found smaller number of
>> early allocated pages.
>> 
>> Now backporting fe53ca54270a on top:
>> 
>> [    0.000000] allocated 738197504 bytes of page_ext
>> [    0.000000] Node 0, zone      DMA: page owner found early allocated 0 pages
>> [    0.000000] Node 0, zone    DMA32: page owner found early allocated 33 pages
>> [    0.000000] Node 0, zone   Normal: page owner found early allocated 2842622 pages
>> [    0.000000] Node 1, zone   Normal: page owner found early allocated 3694362 pages
>> 
>> Again no panic, and same amount of page_ext usage. But the "early allocated" numbers
>> seem bogus to me. I think it's because init_pages_in_zone() is running and inspecting
>> struct pages that have not been yet initialized. It doesn't end up crashing, but
>> still doesn't seem correct?
> 
> Numbers looks sane to me. fe53ca54270a makes init_pages_in_zone()
> called before page_alloc_init_late(). So, there would be many
> uninitialized pages with PageReserved(). Page owner regarded these
> PageReserved() page as allocated page.

That seems incorrect for two reasons:
- init_pages_in_zone() actually skips PageReserved() pages
- the pages don't have PageReserved() flag, until the deferred struct page init
thread processes them via deferred_init_memmap() -> __init_single_page() AFAICS

Now I've found out why upstream reports much less early allocated pages than our
kernel. We're missing 9d43f5aec950 ("mm/page_owner: add zone range overlapping
check") which adds a "page_zone(page) != zone" check. I think this only works
because the pages are not initialized and thus have no nid/zone links. Probably
page_zone() only doesn't break because it's all zeroed. I don't think it's safe
to rely on this?

> We can change the message to "page owner found early reserved N pages"
> 
> Thanks.
>