Re: [PATCH v3] mm: fix panic in __alloc_pages

[Alexey Makhalov <amakhalov@vmware.com>]

> On Dec 8, 2021, at 12:54 AM, Michal Hocko <mhocko@suse.com> wrote:
> 
> Alexey,
> this is still not finalized but it would really help if you could give
> it a spin on your setup. I still have to think about how to transition
> from a memoryless node to standard node (in hotplug code). Also there
> might be other surprises on the way.
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c5952749ad40..8ed8db2ccb13 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6382,7 +6382,11 @@ static void __build_all_zonelists(void *data)
> 	if (self && !node_online(self->node_id)) {
> 		build_zonelists(self);
> 	} else {
> -		for_each_online_node(nid) {
> +		/*
> +		 * All possible nodes have pgdat preallocated
> +		 * free_area_init
> +		 */
> +		for_each_node(nid) {
> 			pg_data_t *pgdat = NODE_DATA(nid);
> 
> 			build_zonelists(pgdat);
> @@ -8032,8 +8036,32 @@ void __init free_area_init(unsigned long *max_zone_pfn)
> 	/* Initialise every node */
> 	mminit_verify_pageflags_layout();
> 	setup_nr_node_ids();
> -	for_each_online_node(nid) {
> -		pg_data_t *pgdat = NODE_DATA(nid);
> +	for_each_node(nid) {
> +		pg_data_t *pgdat;
> +
> +		if (!node_online(nid)) {
> +			pr_warn("Node %d uninitialized by the platform. Please report with boot dmesg.\n", nid);
> +
> +			/* Allocator not initialized yet */
> +			pgdat = memblock_alloc(sizeof(*pgdat), SMP_CACHE_BYTES);
> +			if (!pgdat) {
> +				pr_err("Cannot allocate %zuB for node %d.\n",
> +						sizeof(*pgdat), nid);
> +				continue;
> +			}
> +			/* TODO do we need this for memoryless nodes */
> +			pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
> +			arch_refresh_nodedata(nid, pgdat);
> +			free_area_init_memoryless_node(nid);
> +			/*
> +			 * not marking this node online because we do not want to
> +			 * confuse userspace by sysfs files/directories for node
> +			 * without any memory attached to it (see topology_init)
> +			 */
> +			continue;
> +		}
> +
> +		pgdat = NODE_DATA(nid);
> 		free_area_init_node(nid);
> 
> 		/* Any memory on that node */
> 

Sure Michal, I’ll give it a spin.

Thanks for attention to this topic.

Regarding memory waste. 
Here what I found while was using VM 128 possible NUMA nodes.
My Linux build on VM with only one numa node can be booted on 192Mb RAM,
But on 128 nodes it requires 1GB RAM just to boot. It is server distro,
minimal set of systemd services, no UI.

meminfo shows:
1 node case: Percpu:            53760 kB
128 nodes:   Percpu:           718048 kB !!!

Initial analisys multinode memory consumption showed at least difference in this:

Every memcgroup allocates mem_cgroup_per_node info for all possible node.
Each mem_cgroup_per_node has per cpu stats.
That means, each mem cgroup allocates 128*(sizeof struct mem_cgroup_per_node) + 16384*(sizeof struct lruvec_stats_percpu)

See: mem_cgroup_alloc() -> alloc_mem_cgroup_per_node_info()

There is also old comment about it in alloc_mem_cgroup_per_node_info()
        /*
         * This routine is called against possible nodes.
         * But it's BUG to call kmalloc() against offline node.
         *
         * TODO: this routine can waste much memory for nodes which will
         *       never be onlined. It's better to use memory hotplug callback
         *       function.
         */

Regards,
—Alexey