linux-kernel - Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

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Message-ID: <20180725135147.GN28386@dhcp22.suse.cz>
Date:   Wed, 25 Jul 2018 15:51:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Huang Ying <ying.huang@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Marc-André Lureau 
        <marcandre.lureau@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Miles Chen <miles.chen@...iatek.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Petr Tesarik <ptesarik@...e.cz>
Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

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