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

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Date:   Tue, 24 Jul 2018 14:33:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
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 <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 24.07.2018 14:22, Vlastimil Babka wrote:
> 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.
> 

Had that implemented, Michal didn't like it so far. ("waste of one bit")

-- 

Thanks,

David / dhildenb