linux-kernel - Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock

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Message-ID: <33612969-92a1-6c49-a2e0-3a95715b1e7f@redhat.com>
Date:   Wed, 25 Nov 2020 12:41:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Qian Cai <cai@....pw>, Michal Hocko <mhocko@...nel.org>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set
 pageblock_skip on reserved pages

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