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

[Andrew Morton <akpm@linux-foundation.org>]

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?

> --- 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?