On Tue, Jan 14, 2020 at 01:59:21PM +0300, Kirill A. Shutemov wrote: >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? If my understanding is correct, the reason is deferred_split_huge_page() doesn't has page lock taken, right? > >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 -- Wei Yang Help you, Help me
