On Fri, Oct 16, 2020 at 12:32:50PM +0200, David Hildenbrand wrote:
>>>> Ok, I seems to understand the logic now.
>>>>
>>>> But how we prevent ONLINE_PARTIAL memory block get offlined? There are three
>>>> calls in virtio_mem_set_fake_offline(), while all of them adjust page's flag.
>>>> How they hold reference to struct page?
>>>
>>> Sorry, I should have given you the right pointer. (similar to my other
>>> reply)
>>>
>>> We hold a reference either via
>>>
>>> 1. alloc_contig_range()
>>
>> I am not familiar with this one, need to spend some time to look into.
>
>Each individual page will have a pagecount of 1.
>
>>
>>> 2. memmap init code, when not calling generic_online_page().
>>
>> I may miss some code here. Before online pages, memmaps are allocated in
>> section_activate(). They are supposed to be zero-ed. (I don't get the exact
>> code line.) I am not sure when we grab a refcount here.
>
>Best to refer to __init_single_page() -> init_page_count().
>
>Each page that wasn't onlined via generic_online_page() has a refcount
>of 1 and looks like allocated.
>
Thanks, I see the logic.
online_pages()
move_pfn_range_to_zone() --- 1)
online_pages_range() --- 2)
At 1), __init_single_page() would set page count to 1. At 2),
generic_online_page() would clear page count, while the call back would not.
Then I am trying to search the place where un-zero page count prevent offline.
scan_movable_pages() would fail, since this is a PageOffline() and has 1 page
count.
So the GUARD we prevent offline partial-onlined pages is
(PageOffline && page_count)
And your commit aa218795cb5fd583c94f
mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE
is introduced to handle this case.
That's pretty clear now.
>--
>Thanks,
>
>David / dhildenb
--
Wei Yang
Help you, Help me
