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Date:   Fri, 26 Feb 2021 12:59:00 +0200
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Baoquan He <bhe@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        David Hildenbrand <david@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        =?utf-8?Q?=C5=81ukasz?= Majczak <lma@semihalf.com>,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
        Qian Cai <cai@lca.pw>,
        "Sarvela, Tomi P" <tomi.p.sarvela@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of
 struct page for holes in memory layout
Message-ID: <20210226105900.GK1854360@linux.ibm.com>
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 <20210225180521.GH1854360@linux.ibm.com>
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On Thu, Feb 25, 2021 at 07:38:44PM +0100, Vlastimil Babka wrote:
> On 2/25/21 7:05 PM, Mike Rapoport wrote:
> >> 
> >> What if two zones are adjacent? I.e. if the hole was at a boundary between two
> >> zones.
> > 
> > What do you mean by "adjacent zones"? If there is a hole near the zone
> > boundary, zone span would be clamped to exclude the hole.
> 
> Yeah, zone span should exclude those pages, but you still somehow handle them?
> That's how I read "pages that are not spanned by any node will get links to the
> adjacent zone/node."
> So is it always a unique zone/node can be determined?
> 
> Let's say we have:
> 
> <memory on node 0>
> ---- pageblock boundary ----
> <more memory on node 0>
> <a hole>
> <memory on node 1>
> ---- pageblock boundary ----
> 
> Now I hope such configurations don't really exist :) But if we simulated them in
> QEMU, what would be the linkage in struct pages in that hole?

I don't think such configuration is possible in practice but it can be
forced with e.g memmap="2M hole at 4G - 1M".

The hole in your example the hole will get node1 for node and zone that
spans the beginning of node1 for zone.
 
-- 
Sincerely yours,
Mike.