[PATCH v4 1/5] mm, doc: Add doc for MPOL_F_NUMA_BALANCING

[Yafang Shao <laoar.shao@gmail.com>]

The document on MPOL_F_NUMA_BALANCING was missed in the initial commit
The MPOL_F_NUMA_BALANCING document was inadvertently omitted from the
initial commit bda420b98505 ("numa balancing: migrate on fault among
multiple bound nodes")

Let's ensure its inclusion.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
---
 .../admin-guide/mm/numa_memory_policy.rst          | 27 ++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/admin-guide/mm/numa_memory_policy.rst b/Documentation/admin-guide/mm/numa_memory_policy.rst
index eca38fa..19071b71 100644
--- a/Documentation/admin-guide/mm/numa_memory_policy.rst
+++ b/Documentation/admin-guide/mm/numa_memory_policy.rst
@@ -332,6 +332,33 @@ MPOL_F_RELATIVE_NODES
 	MPOL_PREFERRED policies that were created with an empty nodemask
 	(local allocation).
 
+MPOL_F_NUMA_BALANCING (since Linux 5.12)
+        When operating in MPOL_BIND mode, enables NUMA balancing for tasks,
+        contingent upon kernel support. This feature optimizes page
+        placement within the confines of the specified memory binding
+        policy. The addition of the MPOL_F_NUMA_BALANCING flag augments the
+        control mechanism for NUMA balancing:
+
+        - The sysctl knob numa_balancing governs global activation or
+          deactivation of NUMA balancing.
+
+        - Even if sysctl numa_balancing is enabled, NUMA balancing remains
+          disabled by default for memory areas or applications utilizing
+          explicit memory policies.
+
+        - The MPOL_F_NUMA_BALANCING flag facilitates NUMA balancing
+          activation for applications employing explicit memory policies
+          (MPOL_BIND).
+
+        This flags enables various optimizations for page placement through
+        NUMA balancing. For instance, when an application's memory is bound
+        to multiple nodes (MPOL_BIND), the hint page fault handler attempts
+        to migrate accessed pages to reduce cross-node access if the
+        accessing node aligns with the policy nodemask.
+
+        If the flag isn't supported by the kernel, or is used with mode
+        other than MPOL_BIND, -1 is returned and errno is set to EINVAL.
+
 Memory Policy Reference Counting
 ================================
 
-- 
1.8.3.1