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

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Message-ID: <d14d7a45-91fd-63ef-ea57-513752af1f9e@suse.cz>
Date:   Tue, 24 Jul 2018 14:22:06 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, 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 <willy@...radead.org>,
        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 01:19 PM, Michal Hocko wrote:
>> When creating a crashdump, I definitely need the pages containing memmap
>> included in the dump, so I can inspect the struct pages. But this is a
>> bit recursive issue, so I'll try making it clearer:
>>
>> 1) there are kernel pages with data (e.g. slab) that I typically need in
>> the dump, and are not PageReserved
>> 2) there are struct pages for pages 1) in the memmap that physically
>> hold the pageflags for 1), and these are PageReserved
>> 3) there are struct pages for pages 2) somewhere else in the memmap,
>> physically hold the pageflags for 2). They are probably also
>> PageReserved themselves ? and self-referencing.
>>
>> Excluding PageReserved from dump means there won't be cases 2) and 3) in
>> the dump, which at least for case 2) is making such dump almost useless
>> in many cases.
> 
> Yes, we cannot simply exclude all PageReserved pages. I was merely
> suggesting to rule out new special PageReserved pages that are denoting 
> offline pages. The same could be applied to HWPoison pages

So how about marking them with some "page type" that we got after
Matthew's struct page reorg? I assume the pages we're talking about are
in a state that they don't need the mapcount/mapping field or whatever
unions with the page type... but I guess some care would be needed to
not have false positives when the union field is actually used but
happens to look like the new type.