On 12/7/21 18:44, Andrew Morton 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).
Thanks for the review! I took your changes below into consideration for my V3.
My knowledge of offlined/onlined nodes is quite limited but after looking into
it it doesnt seem like anything clears the state of NODE_DATA(nid) after a
try_offline_node is attempted. So theoretically the panic we saw would not
happen. What is the expected behavior of trying to allocate a page on a offline
node?
>
> 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?
>
>> --- 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?
>
