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



David / dhildenb

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