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

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Date:   Tue, 24 Jul 2018 13:19:13 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
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 <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 11:47:02, Vlastimil Babka wrote:
> On 07/23/2018 02:30 PM, Michal Hocko wrote:
> > On Mon 23-07-18 13:45:18, 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.
> > 
> > You are right. reserve_bootmem_region will make all early bootmem
> > allocations (including those backing memmaps) PageReserved. I have asked
> > several times but I haven't seen a satisfactory answer yet. Why do we
> > even care for kdump about those. If they are reserved the nobody should
> > really look at those specific struct pages and manipulate them. Kdump
> > tools are using a kernel interface to read the content. If the specific
> > content is backed by a non-existing memory then they should simply not
> > return anything.
> 
> 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
-- 
Michal Hocko
SUSE Labs