Re: [PATCH v1 05/12] mm/memory_hotplug: remove nid parameter from remove_memory() and friends

[David Hildenbrand]

On 08.06.21 13:18, David Hildenbrand wrote:
> On 08.06.21 13:11, Michael Ellerman wrote:
> > David Hildenbrand <david@xxxxxxxxxx> 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