Add another layer of fallback policy to make the home node concept
useful from a memory allocation PoV.

This changes the mpol order to:

 - vma->vm_ops->get_policy	[if applicable]
 - vma->vm_policy		[if applicable]
 - task->mempolicy
 - tsk_home_node() preferred	[NEW]
 - default_policy

Note that the tsk_home_node() policy has Migrate-on-Fault enabled to
facilitate efficient on-demand memory migration.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 mm/mempolicy.c |   29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -117,6 +117,22 @@ static struct mempolicy default_policy =
 	.flags = MPOL_F_LOCAL,
 };
 
+static struct mempolicy preferred_node_policy[MAX_NUMNODES];
+
+static struct mempolicy *get_task_policy(struct task_struct *p)
+{
+	struct mempolicy *pol = p->mempolicy;
+	int node;
+
+	if (!pol) {
+		node = tsk_home_node(p);
+		if (node != -1)
+			pol = &preferred_node_policy[node];
+	}
+
+	return pol;
+}
+
 static const struct mempolicy_operations {
 	int (*create)(struct mempolicy *pol, const nodemask_t *nodes);
 	/*
@@ -1478,7 +1494,7 @@ asmlinkage long compat_sys_mbind(compat_
 struct mempolicy *get_vma_policy(struct task_struct *task,
 		struct vm_area_struct *vma, unsigned long addr)
 {
-	struct mempolicy *pol = task->mempolicy;
+	struct mempolicy *pol = get_task_policy(task);
 
 	if (vma) {
 		if (vma->vm_ops && vma->vm_ops->get_policy) {
@@ -1856,7 +1872,7 @@ alloc_pages_vma(gfp_t gfp, int order, st
  */
 struct page *alloc_pages_current(gfp_t gfp, unsigned order)
 {
-	struct mempolicy *pol = current->mempolicy;
+	struct mempolicy *pol = get_task_policy(current);
 	struct page *page;
 
 	if (!pol || in_interrupt() || (gfp & __GFP_THISNODE))
@@ -2302,6 +2318,15 @@ void __init numa_policy_init(void)
 				     sizeof(struct sp_node),
 				     0, SLAB_PANIC, NULL);
 
+	for_each_node(nid) {
+		preferred_node_policy[nid] = (struct mempolicy) {
+			.refcnt = ATOMIC_INIT(1),
+			.mode = MPOL_PREFERRED,
+			.flags = MPOL_F_MOF,
+			.v = { .preferred_node = nid, },
+		};
+	}
+
 	/*
 	 * Set interleaving policy for system init. Interleaving is only
 	 * enabled across suitably sized nodes (default is >= 16MB), or


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