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Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set
 pageblock_skip on reserved pages
From:   David Hildenbrand <david@redhat.com>
To:     Mel Gorman <mgorman@suse.de>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Qian Cai <cai@lca.pw>, Michal Hocko <mhocko@kernel.org>,
        linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
        Baoquan He <bhe@redhat.com>
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 <5f01bde6-fe31-9b0e-f288-06b82598a8b3@redhat.com>
Organization: Red Hat GmbH
Message-ID: <33612969-92a1-6c49-a2e0-3a95715b1e7f@redhat.com>
Date:   Wed, 25 Nov 2020 12:41:55 +0100
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On 25.11.20 12:04, David Hildenbrand wrote:
> On 25.11.20 11:39, Mel Gorman wrote:
>> On Wed, Nov 25, 2020 at 07:45:30AM +0100, David Hildenbrand wrote:
>>>> Something must have changed more recently than v5.1 that caused the
>>>> zoneid of reserved pages to be wrong, a possible candidate for the
>>>> real would be this change below:
>>>>
>>>> +               __init_single_page(pfn_to_page(pfn), pfn, 0, 0);
>>>>
>>>
>>> Before that change, the memmap of memory holes were only zeroed out. So the zones/nid was 0, however, pages were not reserved and had a refcount of zero - resulting in other issues.
>>>
>>> Most pfn walkers shouldn???t mess with reserved pages and simply skip them. That would be the right fix here.
>>>
>>
>> Ordinarily yes, pfn walkers should not care about reserved pages but it's
>> still surprising that the node/zone linkages would be wrong for memory
>> holes. If they are in the middle of a zone, it means that a hole with
>> valid struct pages could be mistaken for overlapping nodes (if the hole
>> was in node 1 for example) or overlapping zones which is just broken.
> 
> I agree within zones - but AFAIU, the issue is reserved memory between
> zones, right?

Double checking, I was confused. This applies also to memory holes
within zones in x86.

-- 
Thanks,

David / dhildenb