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Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC
From:   Qian Cai <cai@lca.pw>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Date:   Tue, 26 Feb 2019 14:19:42 -0500
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On Tue, 2019-02-26 at 19:20 +0100, Michal Hocko wrote:
> Btw. what happens if the offlined pfn range is removed completely? Is
> the range still mapped? What kind of consequences does this have?

Well, the pages are still marked as reserved as well, so it is up to the
physically memory hotplug handler to free kernel direct mapping pagetable,
virtual memory mapping pages, and virtual memory mapping pagetable as by design,
although I have no way to test it.

> Also when does this tweak happens on a completely new hotplugged memory
> range?

I suppose it will call online_pages() which in-turn call
kernel_unmap_linear_page() which may or may not have the same issue, but I have
no way to test that path.