mm/swap: strengthen locking assertions and invariants in cluster allocation

mm/swap: strengthen locking assertions and invariants in cluster allocation

swap_cluster_alloc_table() requires several locks to be held by its
callers: ci->lock, the per-CPU swap_cluster lock, and, for non-solid-state
devices (non-SWP_SOLIDSTATE), the si->global_cluster_lock.

While most call paths (e.g., via cluster_alloc_swap_entry() or
alloc_swap_scan_list()) correctly acquire these locks before invocation,
the path through swap_reclaim_work() -> swap_reclaim_full_clusters() ->
isolate_lock_cluster() is distinct.  This path operates exclusively on
si->full_clusters, where the swap allocation tables are guaranteed to be
already allocated.  Consequently, isolate_lock_cluster() should never
trigger a call to swap_cluster_alloc_table() for these clusters.

Strengthen the locking and state assertions to formalize these invariants:

1. Add a lockdep_assert_held() for si->global_cluster_lock in
   swap_cluster_alloc_table() for non-SWP_SOLIDSTATE devices.
2. Reorder existing lockdep assertions in swap_cluster_alloc_table() to
   match the actual lock acquisition order (per-CPU lock, then global lock,
   then cluster lock).
3. Add a VM_WARN_ON_ONCE() in isolate_lock_cluster() to ensure that table
   allocations are only attempted for clusters being isolated from the
   free list. Attempting to allocate a table for a cluster from other
   lists (like the full list during reclaim) indicates a violation of
   subsystem invariants.

These changes ensure locking consistency and help catch potential
synchronization or logic issues during development.

[zhuhui@kylinos.cn: remove redundant comment, per Barry]
Link: https://lkml.kernel.org/r/20260311022241.177801-1-hui.zhu@linux.dev
[zhuhui@kylinos.cn: initialize `flags', per Chris]
Link: https://lkml.kernel.org/r/20260312023024.903143-1-hui.zhu@linux.dev
Link: https://lkml.kernel.org/r/20260310015657.42395-1-hui.zhu@linux.dev
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Youngjun Park <youngjun.park@lge.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>