Re: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker_info on offlined nodes

[Yang Shi]

On Tue, Dec 7, 2021 at 4:33 PM Nico Pache <npache@xxxxxxxxxx> wrote:
>
>
>
> On 12/7/21 19:26, Yang Shi wrote:
> > On Tue, Dec 7, 2021 at 3:44 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> On Tue,  7 Dec 2021 17:40:13 -0500 Nico Pache <npache@xxxxxxxxxx> wrote:
> >>
> >>> We have run into a panic caused by a shrinker allocation being attempted
> >>> on an offlined node.
> >>>
> >>> Our crash analysis has determined that the issue originates from trying
> >>> to allocate pages on an offlined node in expand_one_shrinker_info. This
> >>> function makes the incorrect assumption that we can allocate on any node.
> >>> To correct this we make sure the node is online before tempting an
> >>> allocation. If it is not online choose the closest node.
> >>
> >> This isn't fully accurate, is it?  We could allocate on a node which is
> >> presently offline but which was previously onlined, by testing
> >> NODE_DATA(nid).
> >>
> >> It isn't entirely clear to me from the v1 discussion why this approach
> >> isn't being taken?
> >>
> >> AFAICT the proposed patch is *already* taking this approach, by having
> >> no protection against a concurrent or subsequent node offlining?
> >
> > AFAICT, we have not reached agreement on how to fix it yet. I saw 3
> > proposals at least:
> >
> > 1. From Michal, allocate node data for all possible nodes.
> > https://lore.kernel.org/all/Ya89aqij6nMwJrIZ@xxxxxxxxxxxxxx/T/#u
> >
> > 2. What this patch does. Proposed originally from
> > https://lore.kernel.org/all/20211108202325.20304-1-amakhalov@xxxxxxxxxx/T/#u
>
> Correct me if im wrong, but isn't that a different caller? This patch fixes the
> issue in expand_one_shrinker_info.

Yes, different caller, but same approach. The cons with this approach
is we have to fix all the places. It seems Michal and David are not
fans for this approach IIRC.

>
> > 3. From David, fix in node_zonelist().
> > https://lore.kernel.org/all/51c65635-1dae-6ba4-daf9-db9df0ec35d8@xxxxxxxxxx/T/#u
> >
> >>
> >>> --- a/mm/vmscan.c
> >>> +++ b/mm/vmscan.c
> >>> @@ -222,13 +222,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> >>>       int size = map_size + defer_size;
> >>>
> >>>       for_each_node(nid) {
> >>> +             int tmp = nid;
> >>
> >> Not `tmp', please.  Better to use an identifier which explains the
> >> variable's use.  target_nid?
> >>
> >> And a newline after defining locals, please.
> >>
> >>>               pn = memcg->nodeinfo[nid];
> >>>               old = shrinker_info_protected(memcg, nid);
> >>>               /* Not yet online memcg */
> >>>               if (!old)
> >>>                       return 0;
> >>>
> >>> -             new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
> >>> +             if(!node_online(nid))
> >>
> >> s/if(/if (/
> >>
> >>> +                     tmp = numa_mem_id();
> >>> +             new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, tmp);
> >>>               if (!new)
> >>>                       return -ENOMEM;
> >>>
> >>
> >> And a code comment fully explaining what's going on here?
> >
>