On 06/28/2017 11:23 AM, Michal Hocko wrote:
> On Mon 26-06-17 11:58:22, Wei Yang wrote:
>> In commit (9adb62a5df9c0fbef7) "mm/hotplug: correctly setup fallback
>> zonelists when creating new pgdat" tries to build the correct zonelist for
>> a new added node, while it is not necessary to rebuild it for already exist
>> nodes.
>>
>> In build_zonelists(), it will iterate on nodes with memory. For a new added
>> node, it will have memory until node_states_set_node() is called in
>> online_pages().
>>
>> This patch will avoid to rebuild the zonelists for already exist nodes.
>
> It is not very clear from the changelog why that actually matters. The
> only effect I can see is that other zonelists on other online nodes will
> not learn about the currently memory less node. This is a good think
> because we do not pointlessly try to allocate from that node.
build_zonelists_node() seems to use managed_zone(zone) checks, so it
should not include empty zones anyway. So effectively seems to me we
just avoid some pointless work under stop_machine().
>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
>> ---
>> mm/page_alloc.c | 16 +++++++++-------
>> 1 file changed, 9 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 560eafe8234d..fc8181b44fd8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5200,15 +5200,17 @@ static int __build_all_zonelists(void *data)
>> memset(node_load, 0, sizeof(node_load));
>> #endif
>>
>> - if (self && !node_online(self->node_id)) {
>> + /* This node is hotadded and no memory preset yet.
>> + * So just build zonelists is fine, no need to touch other nodes.
>> + */
>
> This comment doesn't make much sense to me. What about
> /*
> * Do not rebuild zonelists on all online nodes if the current
> * node is not online yet (it doesn't have any memory) and
> * allocating from it is pointless. Still build zonelist for
> * self because we need to handle memoryless nodes.
> */
>> + if (self && !node_online(self->node_id))
>> build_zonelists(self);
>> - }
>> -
>> - for_each_online_node(nid) {
>> - pg_data_t *pgdat = NODE_DATA(nid);
>> + else
>> + for_each_online_node(nid) {
>> + pg_data_t *pgdat = NODE_DATA(nid);
>>
>> - build_zonelists(pgdat);
>> - }
>> + build_zonelists(pgdat);
>> + }
>>
>> /*
>> * Initialize the boot_pagesets that are going to be used
>> --
>> 2.11.0
>>
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