Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat with node_start_pfn

Michal Hocko
2020-04-21T12:30:18Z

Sorry for the late reply

On Thu 16-04-20 12:47:06, David Hildenbrand wrote:
> A hotadded node/pgdat will span no pages at all, until memory is moved to
> the zone/node via move_pfn_range_to_zone() -> resize_pgdat_range - e.g.,
> when onlining memory blocks. We don't have to initialize the
> node_start_pfn to the memory we are adding.

You are right that the node is empty at this phase but that is already
reflected by zero present pages (hmm, I do not see spanned pages to be
set 0 though). What I am missing here is why this is an improvement. The
new node is already visible here and I do not see why we hide the
information we already know.

> Note: we'll also end up with pgdat->node_start_pfn == 0 when offlined the
> last memory block belonging to a node (via remove_pfn_range_from_zone()->
> update_pgdat_span()).
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> mm/memory_hotplug.c | 15 ++++++---------
> 1 file changed, 6 insertions(+), 9 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 47cf6036eb31..9b15ce465be2 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -866,10 +866,9 @@ static void reset_node_present_pages(pg_data_t *pgdat)
> }
>
> /* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */
> -static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
> +static pg_data_t __ref *hotadd_new_pgdat(int nid)
> {
> struct pglist_data *pgdat;
> - unsigned long start_pfn = PFN_DOWN(start);
>
> pgdat = NODE_DATA(nid);
> if (!pgdat) {
> @@ -899,9 +898,8 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
> }
>
> /* we can use NODE_DATA(nid) from here */
> -
> pgdat->node_id = nid;
> - pgdat->node_start_pfn = start_pfn;
> + pgdat->node_start_pfn = 0;
>
> /* init node's zones as empty zones, we don't have any present pages.*/
> free_area_init_core_hotplug(nid);
> @@ -936,7 +934,6 @@ static void rollback_node_hotadd(int nid)
> /**
> * try_online_node - online a node if offlined
> * @nid: the node ID
> - * @start: start addr of the node
> * @set_node_online: Whether we want to online the node
> * called by cpu_up() to online a node without onlined memory.
> *
> @@ -945,7 +942,7 @@ static void rollback_node_hotadd(int nid)
> * 0 -> the node is already online
> * -ENOMEM -> the node could not be allocated
> */
> -static int __try_online_node(int nid, u64 start, bool set_node_online)
> +static int __try_online_node(int nid, bool set_node_online)
> {
> pg_data_t *pgdat;
> int ret = 1;
> @@ -953,7 +950,7 @@ static int __try_online_node(int nid, u64 start, bool set_node_online)
> if (node_online(nid))
> return 0;
>
> - pgdat = hotadd_new_pgdat(nid, start);
> + pgdat = hotadd_new_pgdat(nid);
> if (!pgdat) {
> pr_err("Cannot online node %d due to NULL pgdat\n", nid);
> ret = -ENOMEM;
> @@ -977,7 +974,7 @@ int try_online_node(int nid)
> int ret;
>
> mem_hotplug_begin();
> - ret = __try_online_node(nid, 0, true);
> + ret = __try_online_node(nid, true);
> mem_hotplug_done();
> return ret;
> }
> @@ -1031,7 +1028,7 @@ int __ref add_memory_resource(int nid, struct resource *res)
> */
> memblock_add_node(start, size, nid);
>
> - ret = __try_online_node(nid, start, false);
> + ret = __try_online_node(nid, false);
> if (ret < 0)
> goto error;
> new_node = ret;
> --
> 2.25.1

--
Michal Hocko
SUSE Labs