# Re: \[PATCH v1 05/12\] mm/memory\_hotplug: remove nid parameter from remove\_memory() and friends

- From: David Hildenbrand \<hidden\>
- Date: 2021-06-09 10:05:11 
- List: linux-mm
- Page: https://ratatoskr.run/linux-mm/2021/06/1408293
- Thread: https://ratatoskr.run/linux-mm/2021/06/1405552/t.md

```text
On 08.06.21 13:18, David Hildenbrand wrote:
> On 08.06.21 13:11, Michael Ellerman wrote:
>> David Hildenbrand [ref] writes:
>>> There is only a single user remaining. We can simply try to offline all
>>> online nodes - which is fast, because we usually span pages and can skip
>>> such nodes right away.
>>
>> That makes me slightly nervous, because our big powerpc boxes tend to
>> trip on these scaling issues before others.
>>
>> But the spanned pages check is just:
>>
>> void try_offline_node(int nid)
>> {
>> 	pg_data_t *pgdat = NODE_DATA(nid);
>>           ...
>> 	if (pgdat->node_spanned_pages)
>> 		return;
>>
>> So I guess that's pretty cheap, and it's only O(nodes), which should
>> never get that big.
>
> Exactly. And if it does turn out to be a problem, we can walk all memory
> blocks before removing them, collecting the nid(s).
>

I might just do the following on top:

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 61bff8f3bfb1..bbc26fdac364 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -2176,7 +2176,9 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 static int check_memblock_offlined_cb(struct memory_block *mem, void *arg)
 {
        int ret = !is_memblock_offlined(mem);
+       int *nid = arg;
 
+       *nid = mem->nid;
        if (unlikely(ret)) {
                phys_addr_t beginpa, endpa;
 
@@ -2271,10 +2273,10 @@ EXPORT_SYMBOL(try_offline_node);
 
 static int __ref try_remove_memory(u64 start, u64 size)
 {
-       int rc = 0, nid;
        struct vmem_altmap mhp_altmap = {};
        struct vmem_altmap *altmap = NULL;
        unsigned long nr_vmemmap_pages;
+       int rc = 0, nid = NUMA_NO_NODE;
 
        BUG_ON(check_hotplug_memory_range(start, size));
 
@@ -2282,8 +2284,12 @@ static int __ref try_remove_memory(u64 start, u64 size)
         * All memory blocks must be offlined before removing memory.  Check
         * whether all memory blocks in question are offline and return error
         * if this is not the case.
+        *
+        * While at it, determine the nid. Note that if we'd have mixed nodes,
+        * we'd only try to offline the last determined one -- which is good
+        * enough for the cases we care about.
         */
-       rc = walk_memory_blocks(start, size, NULL, check_memblock_offlined_cb);
+       rc = walk_memory_blocks(start, size, &nid, check_memblock_offlined_cb);
        if (rc)
                return rc;
 
@@ -2332,7 +2338,7 @@ static int __ref try_remove_memory(u64 start, u64 size)
 
        release_mem_region_adjustable(start, size);
 
-       for_each_online_node(nid)
+       if (nid != NUMA_NO_NODE)
                try_offline_node(nid);
 
        mem_hotplug_done();



-- 
Thanks,

David / dhildenb


```