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

From: Michal Hocko <mhocko@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: "David Hildenbrand" <david@redhat.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Baoquan He" <bhe@redhat.com>, "Dave Young" <dyoung@redhat.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Hari Bathini" <hbathini@linux.vnet.ibm.com>,
	"Huang Ying" <ying.huang@intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Matthew Wilcox" <mawilcox@microsoft.com>,
	"Miles Chen" <miles.chen@mediatek.com>,
	"Pavel Tatashin" <pasha.tatashin@oracle.com>,
	"Petr Tesarik" <ptesarik@suse.cz>
Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps
Date: Tue, 24 Jul 2018 13:19:13 +0200	[thread overview]
Message-ID: <20180724111913.GH28386@dhcp22.suse.cz> (raw)
In-Reply-To: <dca091d3-4c3d-eff5-57f8-a9a45050198d@suse.cz>

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