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

[David Hildenbrand <david@redhat.com>]

On 16.10.19 15:45, Michal Hocko wrote:
> On Wed 16-10-19 14:50:30, David Hildenbrand wrote:
>> On 16.10.19 13:43, Michal Hocko wrote:
>>> On Thu 19-09-19 16:22:25, David Hildenbrand wrote:
>>>> virtio-mem wants to allow to offline memory blocks of which some parts
>>>> were unplugged, especially, to later offline and remove completely
>>>> unplugged memory blocks. The important part is that PageOffline() has
>>>> to remain set until the section is offline, so these pages will never
>>>> get accessed (e.g., when dumping). The pages should not be handed
>>>> back to the buddy (which would require clearing PageOffline() and
>>>> result in issues if offlining fails and the pages are suddenly in the
>>>> buddy).
>>>>
>>>> Let's use "PageOffline() + reference count = 0" as a sign to
>>>> memory offlining code that these pages can simply be skipped when
>>>> offlining, similar to free or HWPoison pages.
>>>>
>>>> Pass flags to test_pages_isolated(), similar as already done for
>>>> has_unmovable_pages(). Use a new flag to indicate the
>>>> requirement of memory offlining to skip over these special pages.
>>>>
>>>> In has_unmovable_pages(), make sure the pages won't be detected as
>>>> movable. This is not strictly necessary, however makes e.g.,
>>>> alloc_contig_range() stop early, trying to isolate such page blocks -
>>>> compared to failing later when testing if all pages were isolated.
>>>>
>>>> Also, make sure that when a reference to a PageOffline() page is
>>>> dropped, that the page will not be returned to the buddy.
>>>>
>>>> memory devices (like virtio-mem) that want to make use of this
>>>> functionality have to make sure to synchronize against memory offlining,
>>>> using the memory hotplug notifier.
>>>>
>>>> Alternative: Allow to offline with a reference count of 1
>>>> and use some other sign in the struct page that offlining is permitted.
>>>
>>> Few questions. I do not see onlining code to take care of this special
>>> case. What should happen when offline && online?
>>
>> Once offline, the memmap is garbage. When onlining again:
>>
>> a) memmap will be re-initialized
>> b) online_page_callback_t will be called for every page in the section. The
>> driver can mark them offline again and not give them to the buddy.
>> c) section will be marked online.
> 
> But we can skip those pages when onlining and keep them in the offline
> state right? We do not poison offlined pages.

https://lkml.org/lkml/2019/10/6/60

But  again, onlining will overwrite the whole memmap right now and there 
is no way to identify if a memmap contains garbage or not.

We would have to identify/remember if re-onlining, but I am not yet sure 
if re-using memmaps when onlining is such a good idea ...

-- 

Thanks,

David / dhildenb