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

[Michal Hocko]

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