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. > http://elixir.free-electrons.com/linux/latest/source/mm/migrate.c#L1576 You mean in the original code? I strongly suspect this was to not take it for each page. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
