[PATCH] mm/mempolicy: Fix decision-making issues for memory migration during NUMA balancing

[Junjie Fu <fujunjie1@qq.com>]

When handling a page fault caused by NUMA balancing (do_numa_page), it is
necessary to decide whether to migrate the current page to another node or
keep it on its current node. For pages with the MPOL_PREFERRED memory
policy, it is sufficient to check whether the first node set in the
nodemask is the same as the node where the page is currently located. If
this is the case, the page should remain in its current state. Otherwise,
migration to another node should be attempted.

Because the definition of MPOL_PREFERRED is as follows: "This mode sets the
preferred node for allocation. The kernel will try to allocate pages from
this node first and fall back to nearby nodes if the preferred node is low
on free memory. If the nodemask specifies more than one node ID, the first
node in the mask will be selected as the preferred node."

Thus, if the node where the current page resides is not the first node in
the nodemask, it is not the PREFERRED node, and memory migration can be
attempted.

However, in the original code, the check only verifies whether the current
node exists in the nodemask (which may or may not be the first node in the
mask). This could lead to a scenario where, if the current node is not the
first node in the nodemask, the code incorrectly decides not to attempt
migration to other nodes.

This behavior is clearly incorrect. If the target node for migration and
the page's current NUMA node are both within the nodemask but neither is
the first node, they should be treated with the same priority, and
migration attempts should proceed.

Signed-off-by: Junjie Fu <fujunjie1@qq.com>
---
 mm/mempolicy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index bb37cd1a51d8..3454dfc7da8d 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2769,7 +2769,7 @@ int mpol_misplaced(struct folio *folio, struct vm_fault *vmf,
 		break;
 
 	case MPOL_PREFERRED:
-		if (node_isset(curnid, pol->nodes))
+		if (curnid == first_node(pol->nodes))
 			goto out;
 		polnid = first_node(pol->nodes);
 		break;
-- 
2.34.1