On Wed, Dec 19, 2018 at 10:57:19AM +0100, Oscar Salvador wrote: >On Wed, Dec 19, 2018 at 10:51:10AM +0100, Michal Hocko wrote: >> On Wed 19-12-18 04:46:56, Wei Yang wrote: >> > Below is a brief call flow for __offline_pages() and >> > alloc_contig_range(): >> > >> > __offline_pages()/alloc_contig_range() >> > start_isolate_page_range() >> > set_migratetype_isolate() >> > drain_all_pages() >> > drain_all_pages() >> > >> > Current logic is: isolate and drain pcp list for each pageblock and >> > drain pcp list again. This is not necessary and we could just drain pcp >> > list once after isolate this whole range. >> > >> > The reason is start_isolate_page_range() will set the migrate type of >> > a range to MIGRATE_ISOLATE. After doing so, this range will never be >> > allocated from Buddy, neither to a real user nor to pcp list. >> >> But it is important to note that those pages still can be allocated from >> the pcp lists until we do drain_all_pages(). > >I had the same fear, but then I saw that move_freepages_block()->move_freepages() moves >the pages to a new list: > ><-- >list_move(&page->lru, > &zone->free_area[order].free_list[migratetype]); >--> > > >But looking at it again, I see that this is only for BuddyPages, so I guess >that pcp-pages do not really get unlinked, so we could still allocate them. Well, I think you are right. But with this in mind, current code looks buggy. Between has_unmovable_pages() and drain_all_pages(), others still could allocate pages on pcp list, right? This means we thought we have isolated the range, but not. So even we do drain_all_pages(), we still missed some pages in this range. > >Uhmf, I missed that. > >-- >Oscar Salvador >SUSE L3 -- Wei Yang Help you, Help me
