linux-kernel - Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat with node_start

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Message-ID: <20200421125250.GG27314@dhcp22.suse.cz>
Date:   Tue, 21 Apr 2020 14:52:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat
 with node_start_pfn

On Tue 21-04-20 14:35:12, David Hildenbrand wrote:
> On 21.04.20 14:30, Michal Hocko wrote:
> > Sorry for the late reply
> > 
> > On Thu 16-04-20 12:47:06, David Hildenbrand wrote:
> >> A hotadded node/pgdat will span no pages at all, until memory is moved to
> >> the zone/node via move_pfn_range_to_zone() -> resize_pgdat_range - e.g.,
> >> when onlining memory blocks. We don't have to initialize the
> >> node_start_pfn to the memory we are adding.
> > 
> > You are right that the node is empty at this phase but that is already
> > reflected by zero present pages (hmm, I do not see spanned pages to be
> > set 0 though). What I am missing here is why this is an improvement. The
> > new node is already visible here and I do not see why we hide the
> > information we already know.
> 
> "information we already know" - no, not before we online the memory.

Is this really the case? All add_memory_resource users operate on a
physical memory range.

> Before onlining, it's just setting node_start_pfn to *some value* to be
> overwritten in move_pfn_range_to_zone()->resize_pgdat_range().

Yes the value is overwritten but I am not sure this is actually correct
thing to do. I cannot remember why I've chosen to do that. It doesn't
really seem unlikely to online node in a higher physical address.

Btw. one thing that I have in my notes, I was never able to actually
test the no numa node case. Because I have always been testing with node
being allocated during the boot. Do you have any way to trigger this
path?
-- 
Michal Hocko
SUSE Labs