On Tue, Jan 14, 2020 at 01:31:12PM +0300, Kirill A. Shutemov wrote: > On Tue, Jan 14, 2020 at 10:31:22AM +0100, Michal Hocko wrote: > > On Sat 11-01-20 03:03:52, Kirill A. Shutemov wrote: > > > On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote: > > > > As all the other places, we grab the lock before manipulate the defer list. > > > > Current implementation may face a race condition. > > > > > > > > For example, the potential race would be: > > > > > > > > CPU1 CPU2 > > > > mem_cgroup_move_account split_huge_page_to_list > > > > !list_empty > > > > lock > > > > !list_empty > > > > list_del > > > > unlock > > > > lock > > > > # !list_empty might not hold anymore > > > > list_del_init > > > > unlock > > > > > > I don't think this particular race is possible. Both parties take page > > > lock before messing with deferred queue, but anytway: > > > > > > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > > > > I am confused, if the above race is not possible then what would be a > > real race? We really do not want to have a patch with a misleading > > changelog, do we? > > The alternative is to make sure that all page_deferred_list() called with > page lock taken. > > I'll look into it. split_huge_page_to_list() has page lock taken. free_transhuge_page() is in the free path and doesn't susceptible to the race. deferred_split_scan() is trickier. list_move() should be safe against list_empty() as it will not produce false-positive list_empty(). list_del_init() *should* (correct me if I'm wrong) be safe because the page is freeing and memcg will not touch the page anymore. deferred_split_huge_page() is a problematic one. It called from page_remove_rmap() path witch does require page lock. I don't see any obvious way to exclude race with mem_cgroup_move_account() here. Anybody else? Wei, could you rewrite the commit message with deferred_split_huge_page() as a race source instead of split_huge_page_to_list()? -- Kirill A. Shutemov
