Re: [PATCH v1 05/29] virtio-mem: generalize check for added memory

From: David Hildenbrand
Date: Sat Oct 17 2020 - 03:39:48 EST



> Am 17.10.2020 um 00:38 schrieb Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>:
>
> 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.
>

I‘m happy to see that I am no longer the only person that understands all this magic :)

Thanks for having a look / reviewing!

>> --
>> Thanks,
>>
>> David / dhildenb
>
> --
> Wei Yang
> Help you, Help me
>