mm, bpf: BPF based THP order selection

Background
----------

Our production servers consistently configure THP to "never" due to
historical incidents caused by its behavior. Key issues include:
- Increased Memory Consumption
  THP significantly raises overall memory usage, reducing available memory
  for workloads.

- Latency Spikes
  Random latency spikes occur due to frequent memory compaction triggered
  by THP.

- Lack of Fine-Grained Control
  THP tuning is globally configured, making it unsuitable for containerized
  environments. When multiple workloads share a host, enabling THP without
  per-workload control leads to unpredictable behavior.

Due to these issues, administrators avoid switching to madvise or always
modes—unless per-workload THP control is implemented.

To address this, we propose BPF-based THP policy for flexible adjustment.
Additionally, as David mentioned [0], this mechanism can also serve as a
policy prototyping tool (test policies via BPF before upstreaming them).

Proposed Solution
-----------------

As suggested by David [0], we introduce a new BPF interface:

/**
 * @get_suggested_order: Get the suggested highest THP order for allocation
 * @mm: mm_struct associated with the THP allocation
 * @tva_flags: TVA flags for current context
 *             %TVA_IN_PF: Set when in page fault context
 *             Other flags: Reserved for future use
 * @order: The highest order being considered for this THP allocation.
 *         %PUD_ORDER for PUD-mapped allocations
 *         %PMD_ORDER for PMD-mapped allocations
 *         %PMD_ORDER - 1 for mTHP allocations
 *
 * Rerurn: Suggested highest THP order to use for allocation. The returned
 * order will never exceed the input @order value.
 */
int (*get_suggested_order)(struct mm_struct *mm, unsigned long tva_flags, int order);

This interface:
- Supports both use cases (per-workload tuning + policy prototyping).
- Can be extended with BPF helpers (e.g., for memory pressure awareness).

This is an experimental feature. To use it, you must enable
CONFIG_EXPERIMENTAL_BPF_ORDER_SELECTION.

Warning:
- The interface may change
- Behavior may differ in future kernel versions
- We might remove it in the future

A simple test case is included in Patch #4.

Changes:
RFC v3->v4:
- Use a new interface get_suggested_order() (David)
- Mark it as experimental (David, Lorenzo)
- Code improvement in THP (Usama)
- Code improvement in BPF struct ops (Amery)

RFC v2->v3: https://lwn.net/Articles/1024545/
- Finer-graind tuning based on madvise or always mode (David, Lorenzo)
- Use BPF to write more advanced policies logic (David, Lorenzo)

RFC v1->v2: https://lwn.net/Articles/1021783/
The main changes are as follows,
- Use struct_ops instead of fmod_ret (Alexei)
- Introduce a new THP mode (Johannes)
- Introduce new helpers for BPF hook (Zi)
- Refine the commit log

RFC v1: https://lwn.net/Articles/1019290/

Yafang Shao (4):
  mm: thp: add support for BPF based THP order selection
  mm: thp: add a new kfunc bpf_mm_get_mem_cgroup()
  mm: thp: add a new kfunc bpf_mm_get_task()
  selftest/bpf: add selftest for BPF based THP order seletection

 include/linux/huge_mm.h                       |  13 +
 include/linux/khugepaged.h                    |  12 +-
 mm/Kconfig                                    |  12 +
 mm/Makefile                                   |   1 +
 mm/bpf_thp.c                                  | 255 ++++++++++++++++++
 mm/huge_memory.c                              |   9 +
 mm/khugepaged.c                               |  18 +-
 mm/memory.c                                   |  14 +-
 tools/testing/selftests/bpf/config            |   2 +
 .../selftests/bpf/prog_tests/thp_adjust.c     | 183 +++++++++++++
 .../selftests/bpf/progs/test_thp_adjust.c     |  69 +++++
 .../bpf/progs/test_thp_adjust_failure.c       |  24 ++
 12 files changed, 605 insertions(+), 7 deletions(-)
 create mode 100644 mm/bpf_thp.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/thp_adjust.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_thp_adjust.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_thp_adjust_failure.c

-- 
2.43.5