Documentation: add BPF-based THP policy management

Add the documentation.

Signed-off-by: Yafang Shao <[email protected]>

Author: laoar

Documentation/admin-guide/mm/transhuge.rst

@@ -738,3 +738,42 @@ support enabled just fine as always. No difference can be noted in
 hugetlbfs other than there will be less overall fragmentation. All
 usual features belonging to hugetlbfs are preserved and
 unaffected. libhugetlbfs will also work fine as usual.
+
+BPF THP
+=======
+
+Overview
+--------
+
+When the system is configured with "always" or "madvise" THP mode, a BPF program
+can be used to adjust THP allocation policies dynamically. This enables
+fine-grained control over THP decisions based on various factors including
+workload identity, allocation context, and system memory pressure.
+
+Program Interface
+-----------------
+
+This feature implements a struct_ops BPF program with the following interface::
+
+  int thp_get_order(struct vm_area_struct *vma,
+                    enum tva_type type,
+                    unsigned long orders);
+
+Parameters::
+
+  @vma: vm_area_struct associated with the THP allocation
+  @type: TVA type for current @vma
+  @orders: Bitmask of available THP orders for this allocation
+
+Return value::
+
+  The suggested THP order for allocation from the BPF program. Must be
+  a valid, available order.
+
+Implementation Notes
+--------------------
+
+This is currently an experimental feature. CONFIG_BPF_THP (EXPERIMENTAL) must be
+enabled to use it. Only one BPF program can be attached at a time, but the
+program can be updated dynamically to adjust policies without requiring affected
+tasks to be restarted.