mm/list_lru: don't pass unnecessary key parameters

mm/list_lru: don't pass unnecessary key parameters

Patch series "Split list_lru lock into per-cgroup scope", v2.

Currently, every list_lru has a per-node lock that protects adding,
deletion, isolation, and reparenting of all list_lru_one instances
belonging to this list_lru on this node.  This lock contention is heavy
when multiple cgroups modify the same list_lru.

This can be alleviated by splitting the lock into per-cgroup scope.

To achieve this, this series reworked and optimized the reparenting
process step by step, making it possible to have a stable list_lru_one,
and making it possible to pin the list_lru_one.  Then split the lock into
per-cgroup scope.

The result is ~15% performance gain for simple multi-cgroup tar test of
small files, and reduced LOC.  See PATCH 5/6 for test details.

This patch (of 6):

When LOCKDEP is not enabled, lock_class_key is an empty struct that is
never used.  But the list_lru initialization function still takes a
placeholder pointer as parameter, and the compiler cannot optimize it
because the function is not static and exported.

Remove this parameter and move it inside the list_lru struct.  Only use it
when LOCKDEP is enabled.  Kernel builds with LOCKDEP will be slightly
larger, while !LOCKDEP builds without it will be slightly smaller (the
common case).

Link: https://lkml.kernel.org/r/20240925171020.32142-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20240925171020.32142-2-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>