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@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 sharing the magic :-) > >Thanks for having a look / reviewing! > >>> -- >>> Thanks, >>> >>> David / dhildenb >> >> -- >> Wei Yang >> Help you, Help me >> -- Wei Yang Help you, Help me
