On 30 Jan 2018, at 11:10, Michal Hocko wrote: > On Tue 30-01-18 10:52:58, Zi Yan wrote: >> >> >> Michal Hocko wrote: >>> On Mon 29-01-18 22:00:11, Zi Yan wrote: >>>> From: Zi Yan <zi.yan@xxxxxxxxxxxxxx> >>>> >>>> migrate_pages() requires at least down_read(mmap_sem) to protect >>>> related page tables and VMAs from changing. Let's do it in >>>> do_page_moves() for both do_move_pages_to_node() and >>>> add_page_for_migration(). >>>> >>>> Also add this lock requirement in the comment of migrate_pages(). >>> >>> This doesn't make much sense to me, to be honest. We are holding >>> mmap_sem for _read_ so we allow parallel updates like page faults >>> or unmaps. Therefore we are isolating pages prior to the migration. >>> >>> The sole purpose of the mmap_sem in add_page_for_migration is to protect >>> from vma going away _while_ need it to get the proper page. >> >> Then, I am wondering why we are holding mmap_sem when calling >> migrate_pages() in existing code. >> https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Felixir.free-electrons.com%2Flinux%2Flatest%2Fsource%2Fmm%2Fmigrate.c%23L1576&data=02%7C01%7Czi.yan%40cs.rutgers.edu%7C855d86d83cff4669d25f08d567fbfb8d%7Cb92d2b234d35447093ff69aca6632ffe%7C1%7C0%7C636529254319323899&sdata=Ba8F7IHIjxDRV%2FeGg7883wlRBmDHQW6pbZubAWZDNLs%3D&reserved=0 > > You mean in the original code? I strongly suspect this was to not take > it for each page. Right. The original code gathers 169 pages, whose information (struct page_to_node, 24bytes) fits into a 4KB page, then migrates them at a time. So mmap_sem is not held for long in the original code, because of this design. I think the question is whether we need to hold mmap_sem for migrate_pages(). Hugh also agrees it is not necessary on a separate email. But it is held in the original code. -- Best Regards Yan Zi
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