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    SubjectRe: [PATCH v2 1/1] mm/vmscan.c: Prevent allocating shrinker_info on offlined nodes
    On Tue, Dec 7, 2021 at 3:44 PM Andrew Morton <akpm@linux-foundation.org> wrote:
    >
    > On Tue, 7 Dec 2021 17:40:13 -0500 Nico Pache <npache@redhat.com> 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@dhcp22.suse.cz/T/#u

    2. What this patch does. Proposed originally from
    https://lore.kernel.org/all/20211108202325.20304-1-amakhalov@vmware.com/T/#u

    3. From David, fix in node_zonelist().
    https://lore.kernel.org/all/51c65635-1dae-6ba4-daf9-db9df0ec35d8@redhat.com/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?

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