linux-kernel - Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG

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Date:   Tue, 26 Feb 2019 15:10:39 -0500
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC

On Tue, 2019-02-26 at 20:40 +0100, Michal Hocko wrote:
> It seems you have missed the point of my question. It simply doesn't
> make much sense to have offline memory mapped. That memory is not
> accessible in general. So mapping it at the offline time is dubious at
> best. 

Well, kernel_map_pages() is like other debug features which could look
"unusual".

> Also you do not get through the offlining phase on a newly
> hotplugged (and not yet onlined) memory. So the patch doesn't look
> correct to me and it all smells like the bug you are seeing is a wrong
> reporting.
> 

That (physical memory hotadd) is a special case like during the boot. The patch
is strictly to deal with offline/online memory, i.e., logical/soft memory
hotplug.