mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems

With class_idx encoded in obj, zs_free() can locate the size_class without
holding pool->lock on 64-bit systems.  Page migration also takes
class->lock and only rewrites the PFN field of obj, so:

  1. read obj locklessly,
  2. lock the size_class derived from obj's class_idx,
  3. re-read obj under class->lock to get a stable PFN.

This eliminates the rwlock read-side cacheline bouncing between zs_free()
and migration/compaction on multi-core systems.

Annotate handle_to_obj()/record_obj() with READ_ONCE()/WRITE_ONCE() to
prevent load/store tearing on the lockless read path and silence KCSAN
data race reports.

When ZS_OBJ_CLASS_BITS == 0 (32-bit, or 64-bit with obj too narrow to hold
class_idx), zs_free() keeps pool->lock.

[akpm@linux-foundation.org: build fix]
[akpm@linux-foundation.org: fix obj_to_class_idx() warning yet again]
[baohua@kernel.org: update the comment about pool lock]
  Link: https://lore.kernel.org/20260725035733.53241-1-baohua@kernel.org
Link: https://lore.kernel.org/20260626015003.2965881-3-haowenchao22@gmail.com
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Xueyuan Chen <xueyuan.chen21@gmail.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions