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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Message-ID: <aeb4aeb9-4ca7-723a-d989-226d537adfb3@suse.cz>
Date:   Fri, 26 Feb 2021 12:08:09 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Rapoport <rppt@...ux.ibm.com>
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 2/26/21 11:59 AM, Mike Rapoport wrote:
> 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".

Right.

> The hole in your example the hole will get node1 for node and zone that
> spans the beginning of node1 for zone.

Yeah the comments in v8 make that clear now, thanks!