Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

[David Hildenbrand]

On 16.10.19 16:09, Michal Hocko wrote:
> On Wed 16-10-19 15:55:00, David Hildenbrand wrote:
> > On 16.10.19 15:45, Michal Hocko wrote:
> [...]
> > > There is state stored in the struct page. In other words this shouldn't
> > > be really different from HWPoison pages. I cannot find the code that is
> > > doing that and maybe we don't handle that. But we cannot simply online
> > > hwpoisoned page. Offlining the range will not make a broken memory OK
> > > all of the sudden. And your usecase sounds similar to me.
> >
> > Sorry to say, but whenever we online memory the memmap is overwritten,
> > because there is no way you could tell it contains garbage or not. You have
> > to assume it is garbage. (my recent patch even poisons the memmap when
> > offlining, which helped to find a lot of these "garbage memmap" BUGs)
> >
> > online_pages()
> > 	...
> > 	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL);
> > 	...
> > 		memmap_init_zone()
> > 			-> memmap initialized
> >
> > So yes, offlining memory with HWPoison and re-onlining it effectively drops
> > HWPoison markers. On the next access, you will trigger a new HWPoison.
>
> Right you are! I need to sit on this much more and think about it with a
> clean head.

Sure, take your time and let me know if you need any details on how 
virtio-mem works, how it uses these interfaces, and how the mm internals 
it uses play along.

Thanks Michal!

--

Thanks,

David / dhildenb