From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Yang Subject: Re: [Patch v2] mm: thp: grab the lock before manipulation defer list Date: Wed, 15 Jan 2020 09:07:22 +0800 Message-ID: <20200115010722.GA4916@richard> References: <20200109143054.13203-1-richardw.yang@linux.intel.com> <20200111000352.efy6krudecpshezh@box> <20200114093122.GH19428@dhcp22.suse.cz> <20200114103112.o6ozdbkfnzdsc2ke@box> <20200114105921.eo2vdwikrvtt3gkb@box> Reply-To: Wei Yang Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20200114105921.eo2vdwikrvtt3gkb@box> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Kirill A. Shutemov" Cc: Michal Hocko , Wei Yang , hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kirill.shutemov-VuQAYsv1563Yd54FQh9/CA@public.gmane.org, yang.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org, alexander.duyck-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org 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 >> > >> > 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