linux-kernel - Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout

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Message-ID: <20210214172906.GN242749@kernel.org>
Date:   Sun, 14 Feb 2021 19:29:06 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     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>,
        "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>,
        Mike Rapoport <rppt@...ux.ibm.com>, Qian Cai <cai@....pw>,
        "Sarvela, Tomi P" <tomi.p.sarvela@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v5 1/1] mm: refactor initialization of struct page for
 holes in memory layout

On Fri, Feb 12, 2021 at 10:56:19AM +0100, David Hildenbrand wrote:
> On 12.02.21 10:55, David Hildenbrand wrote:
> > On 08.02.21 12:08, Mike Rapoport wrote:
> > > +#ifdef CONFIG_SPARSEMEM
> > > +	/*
> > > +	 * Sections in the memory map may not match actual populated
> > > +	 * memory, extend the node span to cover the entire section.
> > > +	 */
> > > +	*start_pfn = round_down(*start_pfn, PAGES_PER_SECTION);
> > > +	*end_pfn = round_up(*end_pfn, PAGES_PER_SECTION);
> > 
> > Does that mean that we might create overlapping zones when one node
> 
> s/overlapping zones/overlapping nodes/
> 
> > starts in the middle of a section and the other one ends in the middle
> > of a section?
> 
> > Could it be a problem? (e.g., would we have to look at neighboring nodes
> > when making the decision to extend, and how far to extend?)

Having a node end/start in a middle of a section would be a problem, but in
this case I don't see a way to detect how a node should be extended :(

We can return to a v4 [1] without x86 modifications.
With that we'll have struct pages corresponding to a hole in a middle of a
zone with correct zone link and a good guess for the node.

As for the pfn 0 on x86, it'll remain outside any node and zone, but since
it was the case since, like forever, I think we can live with it.

[1] https://lore.kernel.org/lkml/20210130221035.4169-1-rppt@kernel.org

-- 
Sincerely yours,
Mike.