linux-kernel - Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

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Date:   Fri, 26 Feb 2021 12:59:00 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        David Hildenbrand <david@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Łukasz Majczak <lma@...ihalf.com>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Qian Cai <cai@....pw>,
        "Sarvela, Tomi P" <tomi.p.sarvela@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of
 struct page for holes in memory layout

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.