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

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Message-ID: <c0df3f96-e8e5-f0eb-2a57-b804a2b0545c@suse.cz>
Date:   Tue, 24 Jul 2018 11:48:00 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        David Hildenbrand <david@...hat.com>
Cc:     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 07/24/2018 09:22 AM, Michal Hocko wrote:
> On Mon 23-07-18 19:12:58, David Hildenbrand wrote:
>> On 23.07.2018 13:45, Vlastimil Babka wrote:
>>> On 07/20/2018 02:34 PM, David Hildenbrand wrote:
>>>> Dumping tools (like makedumpfile) right now don't exclude reserved pages.
>>>> So reserved pages might be access by dump tools although nobody except
>>>> the owner should touch them.
>>>
>>> Are you sure about that? Or maybe I understand wrong. Maybe it changed
>>> recently, but IIRC pages that are backing memmap (struct pages) are also
>>> PG_reserved. And you definitely do want those in the dump.
>>
>> I proposed a new flag/value to mask pages that are logically offline but
>> Michal wanted me to go into this direction.
>>
>> While we can special case struct pages in dump tools ("we have to
>> read/interpret them either way, so we can also dump them"), it smells
>> like my original attempt was cleaner. Michal?
> 
> But we do not have many page flags spare and even if we have one or two
> this doesn't look like the use for them. So I still think we should try
> the PageReserved way.

First we would have to audit everything that's using PageReserved and
might be important for the crash dump to be useful. memmap might not be
the only case...