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Date:   Wed, 25 Nov 2020 13:47:21 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mel Gorman <mgorman@suse.de>, 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>
Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set
 pageblock_skip on reserved pages
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On Wed, Nov 25, 2020 at 12:41:55PM +0100, David Hildenbrand wrote:
> 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.

Yes this is a memory hole within the DMA32 zone.

Still why there should be any difference?

As long as a page struct exists it's in a well defined mem_map array
which comes for one and only one zoneid/nid combination.

So what would be the benefit of treating memory holes within zones or
in between zones differently and leave one or the other with a
zoneid/nid uninitialized?