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

[Michal Hocko <mhocko@kernel.org>]

On Mon 23-07-18 19:20:43, David Hildenbrand wrote:
> On 23.07.2018 14:30, 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.
> > 
> 
> "new kernel" provides an interface to read memory from "old kernel".
> 
> The new kernel has no idea about
> - which memory was added/online in the old kernel
> - where struct pages of the old kernel are and what their content is
> - which memory is save to touch and which not
> 
> Dump tools figure all that out by interpreting the VMCORE. They e.g.
> identify "struct pages" and see if they should be dumped. The "new
> kernel" only allows to read that memory. It cannot hinder to crash the
> system (e.g. if a dump tool would try to read a hwpoison page).
> 
> So how should the "new kernel" know if a page can be touched or not?

I am sorry I am not familiar with kdump much. But from what I remember
it reads from /proc/vmcore and implementation of this interface should
simply return EINVAL or alike when you try to dump inaccessible memory
range.
 
> The *only* way would be to have an interface to the hypervisor where we
> "sense" if a memory location is safe to touch. I remember that xen or
> hyper-v does that - they fake a zero page in that case, after querying
> the hypervisor. But this does not sound like a clean approach to me,
> especially es we need yet another hypervisor interface to sense for
> memory provided via "some" device.
> 
> If we can find a way to just tag pages as "don't touch", it would be the
> easiest and cleanest solution in my opinion.

If only we could have much more spare room in struct pages...
-- 
Michal Hocko
SUSE Labs