Documentation: add BPF THP

Add the documentation.

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

Author: laoar

Documentation/admin-guide/mm/transhuge.rst

@@ -738,3 +738,116 @@ 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
+=======
+
+:Author: Yafang Shao <[email protected]>
+:Date: October 2025
+
+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::
+
+    struct bpf_thp_ops {
+        pid_t pid;
+        thp_order_fn_t *thp_get_order;
+    };
+
+Callback Functions
+------------------
+
+thp_get_order()
+~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+    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 the 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 from the provided ``orders`` bitmask
+
+Operation Modes
+---------------
+
+Per Process Mode
+~~~~~~~~~~~~~~~~
+
+When registering a BPF-THP with a specific PID, the program is installed in the
+target task's ``mm_struct``::
+
+    struct mm_struct {
+        struct bpf_thp_ops __rcu *bpf_thp;
+    };
+
+Inheritance Behavior
+^^^^^^^^^^^^^^^^^^^^
+
+- Existing child processes are unaffected
+- Newly forked children inherit the BPF-THP from their parent
+- The BPF-THP persists across execve() calls
+
+Management Rules
+^^^^^^^^^^^^^^^^
+
+- When a BPF-THP instance is unregistered, all managed tasks' ``bpf_thp``
+  pointers are reset to ``NULL``
+- When a BPF-THP instance is updated, all managed tasks' ``bpf_thp`` pointers
+  are automatically updated to the new version
+- Each process can be managed by only one BPF-THP instance at a time
+
+Global Mode
+~~~~~~~~~~~
+
+If no PID is specified during registration, the BPF-THP operates in global mode.
+In this mode, all tasks in the system are managed by the global instance.
+
+Global Mode Precedence
+^^^^^^^^^^^^^^^^^^^^^^
+
+- The global instance takes precedence over all per-process instances
+- All existing per-process instances are disabled when a global instance is
+  registered
+- New per-process registrations are blocked while a global instance is active
+- Existing per-process instances remain registered (no forced unregistration)
+
+Instance Management
+^^^^^^^^^^^^^^^^^^^
+
+- Updates are type-isolated: global instances can only be updated by new global
+  instances, and per-process instances by new per-process instances
+- Only one global BPF-THP can be registered at a time
+- Global instances can be updated dynamically without requiring task restarts
+
+Implementation Notes
+--------------------
+
+- This is currently an experimental feature
+- ``CONFIG_BPF_THP`` must be enabled to use this functionality
+- The feature depends on proper THP configuration ("always" or "madvise" mode)