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

[Michal Hocko <mhocko@kernel.org>]

On Wed 25-07-18 16:20:41, David Hildenbrand wrote:
> On 25.07.2018 15:51, Michal Hocko wrote:
> > On Tue 24-07-18 16:13:09, David Hildenbrand wrote:
> > [...]
> >> So I see right now:
> >>
> >> - Pg_reserved + e.g. new page type (or some other unique identifier in
> >>   combination with Pg_reserved)
> >>  -> Avoid reads of pages we know are offline
> >> - extend is_ram_page()
> >>  -> Fake zero memory for pages we know are offline
> >>
> >> Or even both (avoid reading and don't crash the kernel if it is being done).
> > 
> > I really fail to see how that can work without kernel being aware of
> > PageOffline. What will/should happen if you run an old kdump tool on a
> > kernel with this partially offline memory?
> > 
> 
> New kernel with old dump tool:
> 
> a) we have not fixed up is_ram_page()
> 
> -> crash, as we access memory we shouldn't

this is not acceptable, right? You do not want to crash your crash
kernel ;)

> b) we have fixed up is_ram_page()
> 
> -> We have a callback to check for applicable memory in the hypervisor
> whether the parts are accessible / online or not accessible / offline.
> (e.g. via a device driver that controls a certain memory region)
> 
> -> Don't read, but fake a page full of 0
> 
> 
> So instead of the kernel being aware of it, it asks via is_ram_page()
> the hypervisor.

I am still confused why do we even care about hypervisor. What if
somebody wants to have partial memory hotplug on native OS?
 
> I don't think a) is a problem. AFAICS, we have to update makedumpfile
> for every new kernel. We can perform changes and update makedumpfile
> to be compatible with new dump tools.

Not really. You simply do not crash the kernel just because you are
trying to dump the already crashed kernel.

> E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump
> tools and required

But has it crashed the kernel when reading the dump? If yes then the
whole dumping is fragile as hell...
-- 
Michal Hocko
SUSE Labs