linux-kernel - Re: [PATCH] mm/spase: never partially remove memmap for early section

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Message-ID: <20200624085015.GG1320@dhcp22.suse.cz>
Date:   Wed, 24 Jun 2020 10:50:15 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, osalvador@...e.de,
        dan.j.williams@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section

On Wed 24-06-20 10:41:18, David Hildenbrand wrote:
[...]
> But nothing actually breaks .... because *drummroll* we use huge pages in the vmemmap,
> so the partial depopulate will not actually depopulate anything here. Huge page is 2M,
> the memmap of 128MB sections is exactly 2MB == one hugepages. Trying to depopulate a
> fraction (e.g., 16MB) of that won't do anything.
> 
> 
> Now, forcing a CPU without hugepages - PSE (QEMU: "-cpu host,pse=off)", I can trigger
> via  ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M

OK, now I am following! Thanks a lot. That is something to be mentioned
in the changelog. Small pages might be used if the hotplug happens on a
system with fragmented memory so this is something we do care about.

Thanks a lot David!

-- 
Michal Hocko
SUSE Labs