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    SubjectRe: [PATCH v1 05/29] virtio-mem: generalize check for added memory
    On Sat, Oct 17, 2020 at 09:39:38AM +0200, David Hildenbrand wrote:
    >
    >> Am 17.10.2020 um 00:38 schrieb Wei Yang <richard.weiyang@linux.alibaba.com>:
    >>
    >> 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 sharing the magic :-)

    >
    >Thanks for having a look / reviewing!
    >
    >>> --
    >>> Thanks,
    >>>
    >>> David / dhildenb
    >>
    >> --
    >> Wei Yang
    >> Help you, Help me
    >>

    --
    Wei Yang
    Help you, Help me

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