[PATCH v4 0/2] Make KHO Stateless

[Jason Miu <jasonmiu@google.com>]

This series transitions KHO from an xarray-based metadata tracking system
with serialization to a radix tree data structure that can be passed
directly to the next kernel.

The key motivations for this change are to:
- Eliminate the need for data serialization before kexec.
- Remove the KHO finalize state.
- Pass preservation metadata more directly to the next kernel via the FDT.

The new approach uses a radix tree to mark preserved pages. A page's
physical address and its order are encoded into a single value. The tree
is composed of multiple levels of page-sized tables, with leaf nodes
being bitmaps where each set bit represents a preserved page. The
physical address of the radix tree's root is passed in the FDT, allowing
the next kernel to reconstruct the preserved memory map.

This series is broken down into the following patches:

1.  kho: Adopt radix tree for preserved memory tracking:    
    Replaces the xarray-based tracker with the new radix tree
    implementation and increments the ABI version.

2.  kho: Remove finalize state and clients:
    Removes the now-obsolete kho_finalize() function and its usage
    from client code and debugfs.

---

Changelog since v3 [1]:
- The patches introducing the KHO FDT ABI header [2] and relocating the
  vmalloc preservation structure to the KHO ABI header [3] were merged
  into another series [4].
- Use `struct kho_radix_tree` to encapsulate the tree structure and
  expose it to the public radix tree APIs as an argument.
- Public radix tree APIs now manage the tree structure's lock internally.
- Protect the radix tree with a mutex lock instead of a read-write
  semaphore.
- Radix tree root pointer validation and warnings are now centralized in
  kho_radix_add_page() and kho_radix_del_page().
- Updates to the KHO finalization logic are grouped into the second patch.
- kho_radix_encode_key() and kho_radix_decode_key() were removed from
  the public APIs.
- KHO radix tree callback function takes physical address and order of a page as
  inputs instead of a radix key.
- Refactored kho_radix_get_index() to use kho_radix_get_bitmap_index().
- Updated the documentation.

[1] https://lore.kernel.org/lkml/20251209025317.3846938-1-jasonmiu@google.com/
[2] https://lore.kernel.org/lkml/20251209025317.3846938-2-jasonmiu@google.com/
[3] https://lore.kernel.org/lkml/20251209025317.3846938-3-jasonmiu@google.com/
[4] https://lore.kernel.org/lkml/20260105165839.285270-1-rppt@kernel.org/

---

Jason Miu (2):
  kho: Adopt radix tree for preserved memory tracking
  kho: Remove finalize state and clients

 Documentation/admin-guide/mm/kho.rst        |  52 +-
 Documentation/core-api/kho/abi.rst          |   6 +
 Documentation/core-api/kho/index.rst        |  23 +-
 include/linux/kho/abi/kexec_handover.h      | 141 +++-
 include/linux/kho_radix_tree.h              |  72 ++
 kernel/liveupdate/kexec_handover.c          | 703 ++++++++++----------
 kernel/liveupdate/kexec_handover_debugfs.c  |  23 -
 kernel/liveupdate/kexec_handover_internal.h |   3 -
 kernel/liveupdate/luo_core.c                |  12 +-
 kernel/liveupdate/luo_flb.c                 |   2 +-
 tools/testing/selftests/kho/init.c          |  20 -
 11 files changed, 573 insertions(+), 484 deletions(-)
 create mode 100644 include/linux/kho_radix_tree.h


base-commit: f96074c6d01d8a5e9e2fccd0bba5f2ed654c1f2d
-- 
2.52.0.457.g6b5491de43-goog