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

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Date:   Tue, 21 Apr 2020 14:35:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
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 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.

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

(I have some more patches to clean up __try_online_node(), and this
change here makes it clear that there isn't any magic happening in
__try_online_node() related to the start pfn and memblocks - see patch #2)

-- 
Thanks,

David / dhildenb