Re: [PATCH 4/6] mm, memory_hotplug: reorganize new pgdat initialization

David Hildenbrand
2022-01-27T12:47:00Z

On 27.01.22 09:53, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> When a !node_online node is brought up it needs a hotplug specific
> initialization because the node could be either uninitialized yet or it
> could have been recycled after previous hotremove. hotadd_init_pgdat is
> responsible for that.
>
> Internal pgdat state is initialized at two places currently
> - hotadd_init_pgdat
> - free_area_init_core_hotplug
> There is no real clear cut what should go where but this patch's chosen to
> move the whole internal state initialization into free_area_init_core_hotplug.
> hotadd_init_pgdat is still responsible to pull all the parts together -
> most notably to initialize zonelists because those depend on the overall topology.
>
> This patch doesn't introduce any functional change.
>
> Acked-by: Rafael Aquini <raquini@redhat.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> include/linux/memory_hotplug.h | 2 +-
> mm/memory_hotplug.c | 28 +++-------------------------
> mm/page_alloc.c | 25 +++++++++++++++++++++++--
> 3 files changed, 27 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 60f09d3ebb3d..76bf2de86def 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -319,7 +319,7 @@ extern void set_zone_contiguous(struct zone *zone);
> extern void clear_zone_contiguous(struct zone *zone);
>
> #ifdef CONFIG_MEMORY_HOTPLUG
> -extern void __ref free_area_init_core_hotplug(int nid);
> +extern void __ref free_area_init_core_hotplug(struct pglist_data *pgdat);
> extern int __add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
> extern int add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
> extern int add_memory_resource(int nid, struct resource *resource,
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 875cdc7ffa58..ddc62f8b591f 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1166,39 +1166,16 @@ static pg_data_t __ref *hotadd_init_pgdat(int nid)
> {
> struct pglist_data *pgdat;
>
> - pgdat = NODE_DATA(nid);
> -
> /*
> * NODE_DATA is preallocated (free_area_init) but its internal
> * state is not allocated completely. Add missing pieces.
> * Completely offline nodes stay around and they just need
> * reintialization.
> */
> - if (pgdat->per_cpu_nodestats == &boot_nodestats) {
> - pgdat->per_cpu_nodestats =
> - alloc_percpu(struct per_cpu_nodestat);
> - } else {
> - int cpu;
> - /*
> - * Reset the nr_zones, order and highest_zoneidx before reuse.
> - * Note that kswapd will init kswapd_highest_zoneidx properly
> - * when it starts in the near future.
> - */
> - pgdat->nr_zones = 0;
> - pgdat->kswapd_order = 0;
> - pgdat->kswapd_highest_zoneidx = 0;
> - for_each_online_cpu(cpu) {
> - struct per_cpu_nodestat *p;
> -
> - p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
> - memset(p, 0, sizeof(*p));
> - }
> - }
> -
> - pgdat->node_start_pfn = 0;
> + pgdat = NODE_DATA(nid);
>
> /* init node's zones as empty zones, we don't have any present pages.*/
> - free_area_init_core_hotplug(nid);
> + free_area_init_core_hotplug(pgdat);
>
> /*
> * The node we allocated has no zone fallback lists. For avoiding
> @@ -1210,6 +1187,7 @@ static pg_data_t __ref *hotadd_init_pgdat(int nid)
> * When memory is hot-added, all the memory is in offline state. So
> * clear all zones' present_pages because they will be updated in
> * online_pages() and offline_pages().
> + * TODO: should be in free_area_init_core_hotplug?
> */
> reset_node_managed_pages(pgdat);
> reset_node_present_pages(pgdat);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1a05669044d3..32d0189de4c5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7506,12 +7506,33 @@ static void __meminit zone_init_internals(struct zone *zone, enum zone_type idx,
> * NOTE: this function is only called during memory hotplug
> */
> #ifdef CONFIG_MEMORY_HOTPLUG
> -void __ref free_area_init_core_hotplug(int nid)
> +void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
> {
> + int nid = pgdat->node_id;
> enum zone_type z;
> - pg_data_t *pgdat = NODE_DATA(nid);
> + int cpu;
>
> pgdat_init_internals(pgdat);
> +
> + if (pgdat->per_cpu_nodestats == &boot_nodestats)
> + pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
> +
> + /*
> + * Reset the nr_zones, order and highest_zoneidx before reuse.
> + * Note that kswapd will init kswapd_highest_zoneidx properly
> + * when it starts in the near future.
> + */
> + pgdat->nr_zones = 0;
> + pgdat->kswapd_order = 0;
> + pgdat->kswapd_highest_zoneidx = 0;
> + pgdat->node_start_pfn = 0;
> + for_each_online_cpu(cpu) {
> + struct per_cpu_nodestat *p;
> +
> + p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
> + memset(p, 0, sizeof(*p));
> + }
> +
> for (z = 0; z < MAX_NR_ZONES; z++)
> zone_init_internals(&pgdat->node_zones[z], z, nid, 0);
> }

I feel like we should be initializing all of this only once, just after
allocating the node. There should be no difference between a node we're
reusing and a "fresh" node. IOW, memory offlining should be resetting
all state accordingly when the last memory goes away.

But I might be wrong and this patch looks like an improvement, as you
say, without functional change

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb