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Date: Wed, 25 Jul 2018 15:51:47 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps
Message-ID: <20180725135147.GN28386@dhcp22.suse.cz>
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To: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, Baoquan He <bhe@redhat.com>, Dave Young <dyoung@redhat.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Hari Bathini <hbathini@linux.vnet.ibm.com>, Huang Ying <ying.huang@intel.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, =?iso-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@redhat.com>, Matthew Wilcox <mawilcox@microsoft.com>, Miles Chen <miles.chen@mediatek.com>, Pavel Tatashin <pasha.tatashin@oracle.com>, Petr Tesarik <ptesarik@suse.cz>

On Tue 24-07-18 16:13:09, David Hildenbrand wrote:
[...]
> So I see right now:
> 
> - Pg_reserved + e.g. new page type (or some other unique identifier in
>   combination with Pg_reserved)
>  -> Avoid reads of pages we know are offline
> - extend is_ram_page()
>  -> Fake zero memory for pages we know are offline
> 
> Or even both (avoid reading and don't crash the kernel if it is being done).

I really fail to see how that can work without kernel being aware of
PageOffline. What will/should happen if you run an old kdump tool on a
kernel with this partially offline memory?
-- 
Michal Hocko
SUSE Labs