On 22 Nov 2017, at 7:13, Zi Yan wrote: > On 22 Nov 2017, at 5:14, Michal Hocko wrote: > >> On Wed 22-11-17 10:35:10, Michal Hocko wrote: >> [...] >>> Moreover I am not really sure this is really working properly. Just look >>> at the split_huge_page. It moves all the tail pages to the LRU list >>> while migrate_pages has a list of pages to migrate. So we will migrate >>> the head page and all the rest will get back to the LRU list. What >>> guarantees that they will get migrated as well. >> >> OK, so this is as I've expected. It doesn't work! Some pfn walker based >> migration will just skip tail pages see madvise_inject_error. >> __alloc_contig_migrate_range will simply fail on THP page see >> isolate_migratepages_block so we even do not try to migrate it. >> do_move_page_to_node_array will simply migrate head and do not care >> about tail pages. do_mbind splits the page and then fall back to pte >> walk when thp migration is not supported but it doesn't handle tail >> pages if the THP migration path is not able to allocate a fresh THP >> AFAICS. Memory hotplug should be safe because it doesn't skip the whole >> THP when doing pfn walk. >> >> Unless I am missing something here this looks like a huge mess to me. > > +Kirill > > First, I agree with you that splitting a THP and only migrating its head page > is a mess. But what you describe is also the behavior of migrate_page() > _before_ THP migration support is added. I thought that was intended. > > Look at http://elixir.free-electrons.com/linux/v4.13.15/source/mm/migrate.c#L1091, > unmap_and_move() splits THPs and only migrates the head page in v4.13 before THP > migration is added. I think the behavior was introduced since v4.5 (I just skimmed > v4.0 to v4.13 code and did not have time to use git blame), before that THPs are > not migrated but shown as successfully migrated (at least from v4.4’s code). Sorry, I misread v4.4’s code, it also does ‘splitting a THP and migrating its head page’. This behavior was there for a long time, at least since v3.0. The code in unmap_and_move() is: if (unlikely(PageTransHuge(page))) if (unlikely(split_huge_page(page))) goto out; Hope I did not miss anything else. — Best Regards, Yan Zi
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