On Wed, Dec 19, 2018 at 03:13:43PM +0100, Michal Hocko wrote: >On Wed 19-12-18 13:53:07, Wei Yang wrote: >> 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. > >THere is no guarantee in that regards and I believe there is also no >demand for such a strong semantic. Or I do not see it at least. If there is no demand for this, allocating page before drain_all_pages() is reasonable. The time gap between isolating page and drain_all_pages is fine. Then I am confused about the objection to this patch. Finally, we drain all the pages in pcp list and the range is isolated. >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
