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Date:   Tue, 26 Feb 2019 21:40:47 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC
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On Tue 26-02-19 15:10:39, Qian Cai wrote:
> 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.

And it doesn't handle it properly AFAICS. You want to get an exception
when accessing an offline memory, don't you? Offline, free or not present
memory is basically the same case - nobody should be touching that
memory.

-- 
Michal Hocko
SUSE Labs