Re: [PATCH] mm/memory-tiers: reset demotion targets of nodes without memory

[Gregory Price <gourry@gourry.net>]

On Mon, Oct 19, 2026 at 03:04:45PM -0400, Zi Yan wrote:
> On Mon Oct 19, 2026 at 2:47 PM EDT, Gregory Price (Meta) wrote:
> > When a node loses its last memory, offline_pages() clears N_MEMORY
> > before the memory-tier callback rebuilds the demotion targets.
> > disable_all_demotion_targets() resets only N_MEMORY nodes, so the node
> > keeps its old node_demotion[] entry while it has no memory. Before
> > 6c542ab75714, the reset covered every online node.
> 
> establish_demotion_targets() uses for_each_node_state(node, N_MEMORY),
> because the to-be-online node is already added into N_MEMORY?
> 

NODE_ADDED_FIRST_MEMORY happens after the memory is already added
(though not necessarily online). 

NODE_REMOVED_LAST_MEMORY happens after the last memory block is removed
(and therefore N_MEMORY is gone).

slightly different timing for each event.  There isn't a reasonable
change we can make otherwise.

> >
> > A reclaimer reads the stale entry only between the node regaining memory
> > and its targets being rebuilt, and it filters the entry against the
> > current allowed targets, so there is no user-visible effect. To see it,
> > online and then offline a node's only DIMM: a debugger still finds the
> > lower-tier node in node_demotion[nid].preferred.
> >
> > Reset the targets of every node.
> >
> > Fixes: 6c542ab75714 ("mm/demotion: build demotion targets based on explicit memory tiers")
> > Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> > ---
> >  mm/memory-tiers.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> > index 25e121851b586..7197f1a67a340 100644
> > --- a/mm/memory-tiers.c
> > +++ b/mm/memory-tiers.c
> > @@ -378,7 +378,7 @@ static void disable_all_demotion_targets(void)
> >  	struct memory_tier *memtier;
> >  	int node;
> >  
> > -	for_each_node_state(node, N_MEMORY) {
> > +	for_each_node(node) {
> 
> I think the above code deserves a comment explaining why for_each_node()
> is needed instead of for_each_node_state(N_MEMORY), because when a node
> is added or removed during memory hotadd or hotremove matters here.
> 

Sure.  Will respin tomorrow and give others a chance to look at it.

~Gregory