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Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set
 pageblock_skip on reserved pages
To:     Andrea Arcangeli <aarcange@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     Mel Gorman <mgorman@suse.de>,
        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>
References: <8C537EB7-85EE-4DCF-943E-3CC0ED0DF56D@lca.pw>
 <20201121194506.13464-1-aarcange@redhat.com>
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 <ea911b11-945f-d2c5-5558-a3fe0bda492a@suse.cz> <X73s8fxDKPRD6wET@redhat.com>
 <1c4c405b-52e0-cf6b-1f82-91a0a1e3dd53@suse.cz> <X76qJCg0Pa8diO59@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <424bba1c-d9af-e2a9-0b5a-3a71d525bf30@redhat.com>
Date:   Wed, 25 Nov 2020 20:33:01 +0100
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On 25.11.20 20:01, Andrea Arcangeli wrote:
> On Wed, Nov 25, 2020 at 01:08:54PM +0100, Vlastimil Babka wrote:
>> Yeah I guess it would be simpler if zoneid/nid was correct for 
>> pfn_valid() pfns within a zone's range, even if they are reserved due 
>> not not being really usable memory.
>>
>> I don't think we want to introduce CONFIG_HOLES_IN_ZONE to x86. If the 
>> chosen solution is to make this to a real hole, the hole should be 
>> extended to MAX_ORDER_NR_PAGES aligned boundaries.
> 
> The way pfn_valid works it's not possible to render all non-RAM pfn as
> !pfn_valid, CONFIG_HOLES_IN_ZONE would not achieve it 100% either. So

Well, we could do it the arm64 way and provide a custom pfn_valid() and
check memblock for RAM - please don't! :D

> I don't think we can rely on that to eliminate all non-RAM reserved
> pages from the mem_map and avoid having to initialize them in the
> first place. Some could remain as in this case since in the same
> pageblock there's non-RAM followed by RAM and all pfn are valid.
> 
>> In any case, compaction code can't fix this with better range checks.
> 
> David's correct that it can, by adding enough PageReserved (I'm
> running all systems reproducing this with plenty of PageReserved
> checks in all places to work around it until we do a proper fix).
> 
> My problem with that is that 1) it's simply non enforceable at runtime
> that there is not missing PageReserved check and 2) what benefit it
> would provide to leave a wrong zoneid in reserved pages and having to
> add extra PageReserved checks?

See my other mail. If we have a clean way to set *any* memmap (non-RAM,
memory holes at any place) to a proper nid/zid, then we won't need
reserved checks. I raised some cases that need more thought than a
simple "hole in zone".


-- 
Thanks,

David / dhildenb