Re: [PATCH v5 05/36] mm: add ZONELIST_PRIVATE(_NOFALLBACK) for N_MEMORY_PRIVATE nodes.

[Gregory Price <gourry@gourry.net>]

On Wed, Jul 22, 2026 at 09:00:51PM +0800, Richard Cheng wrote:
> On Mon, Jul 20, 2026 at 03:33:59PM +0800, Gregory Price wrote:
> >  static void build_zonelists(pg_data_t *pgdat)
> >  {
> >  	static int node_order[MAX_NUMNODES];
> > +	nodemask_t tier_nodes;
> >  	int local_node = pgdat->node_id;
> >  	int node, nr_nodes = 0;
> >  
> >  	memset(node_order, 0, sizeof(node_order));
> >  
> > +	/*
> > +	 * Zonelists: FALLBACK, NOFALLBACK, PRIVATE
> > +	 *
> > +	 * FALLBACK:   Allocation order for all nodes.  Private nodes have lists
> > +	 *             but never appear as an entry in any list. Allocations
> > +	 *             targeting a private node w/ FALLBACK land on N_MEMORY.
> > +	 *
> > +	 * NOFALLBACK: A list for each node containing only itself.
> > +	 *             Allocations targeting a private node w/ NOFALLBACK
> > +	 *             will always fail (their NOFALLBACK is empty)
> > +	 *
> > +	 * PRIVATE:    (N_MEMORY | N_MEMORY_PRIVATE) - allows access to
> > +	 *             private nodes, and falls back to normal memory unless
> > +	 *             __GFP_THISNODE otherwise constrains it.
> > +	 */
> > +
> >  	build_node_zonelist(pgdat, &node_states[N_MEMORY], ZONELIST_FALLBACK,
> >  			    true, node_order, &nr_nodes);
> >  	build_thisnode_zonelists(pgdat);
> > @@ -5912,6 +5957,10 @@ static void build_zonelists(pg_data_t *pgdat)
> >  	for (node = 0; node < nr_nodes; node++)
> >  		pr_cont("%d ", node_order[node]);
> >  	pr_cont("\n");
> > +
> > +	nodes_or(tier_nodes, node_states[N_MEMORY], node_states[N_MEMORY_PRIVATE]);
> 
> Hi Gregory,
> 
> A question about this part, if my understanding is right,
> ZONELIST_PRIVATE contains all private nodes, should we use some explicit target
> nodemask here to make sure only the targeting owner's private node can be touched?
> 
> Otherwise it might fall back into a different owner's private node.
> 
> --Richard
> 

Zonelists and the constructors here is a bit difficult to understand,
and I see I actually did a poor job of explaining what's going on in
the comments here.  I will fix that up in v6.

Lets say we have the following nodes:
   N_MEMORY :  0 1
   N_MEMORY_PRIVATE : 2 3

The following zonelists would exist:

ZONELIST_FALLBACK[0]   = {0,1}
ZONELIST_FALLBACK[1]   = {1,0}
ZONELIST_FALLBACK[2]   = {0,1}
ZONELIST_FALLBACK[3]   = {0,1}
ZONELIST_NOFALLBACK[0] = {0}
ZONELIST_NOFALLBACK[1] = {1}
ZONELIST_NOFALLBACK[2] = {}
ZONELIST_NOFALLBACK[3] = {}
ZONELIST_PRIVATE[0]    = {0,1}
ZONELIST_PRIVATE[1]    = {1,0}
ZONELIST_PRIVATE[2]    = {2,0,1}  <- no fallback to 3
ZONELIST_PRIVATE[3]    = {3,0,1}  <- no fallback to 2
ZONELIST_PRIVATE_NOFALLBACK[0] = {0}
ZONELIST_PRIVATE_NOFALLBACK[1] = {1}
ZONELIST_PRIVATE_NOFALLBACK[2] = {2} <- __GFP_THISNODE
ZONELIST_PRIVATE_NOFALLBACK[3] = {3} <- __GFP_THISNODE

In the current design, we actually can never fallback to another private
node, and instead if you wanted to - for example - interleave across
said nodes you'd have to do so manually.

I think it's reasonable to later extend zonelist construction to support
such a think like:

ZONELIST_PRIVATE[2] = {2,0,1,3}

Later if such a need arises, but I would say wait for the use case.

~Gregory