mm: memcg: introduce memcontrol-v1.c

mm: memcg: introduce memcontrol-v1.c

Patch series "mm: memcg: separate legacy cgroup v1 code and put under
config option", v2.

Cgroups v2 have been around for a while and many users have fully adopted
them, so they never use cgroups v1 features and functionality.  Yet they
have to "pay" for the cgroup v1 support anyway:
1) the kernel binary contains an unused cgroup v1 code,
2) some code paths have additional checks which are not needed,
3) some common structures like task_struct and mem_cgroup contain unused
   cgroup v1-specific members.

Cgroup v1's memory controller has a number of features that are not
supported by cgroup v2 and their implementation is pretty much self
contained.  Most notably, these features are: soft limit reclaim, oom
handling in userspace, complicated event notification system, charge
migration.  Cgroup v1-specific code in memcontrol.c is close to 4k lines
in size and it's intervened with generic and cgroup v2-specific code.
It's a burden on developers and maintainers.

This patchset aims to solve these problems by:
1) moving cgroup v1-specific memcg code to the new mm/memcontrol-v1.c file,
2) putting definitions shared by memcontrol.c and memcontrol-v1.c into the
   mm/memcontrol-v1.h header,
3) introducing the CONFIG_MEMCG_V1 config option, turned off by default,
4) making memcontrol-v1.c to compile only if CONFIG_MEMCG_V1 is set.

If CONFIG_MEMCG_V1 is not set, cgroup v1 memory controller is still available
for mounting, however no memory-specific control knobs are present.

This patch (of 14):

This patch introduces the mm/memcontrol-v1.c source file which will be
used for all legacy (cgroup v1) memory cgroup code.  It also introduces
mm/memcontrol-v1.h to keep declarations shared between mm/memcontrol.c and
mm/memcontrol-v1.c.

As of now, let's compile it if CONFIG_MEMCG is set, similar to
mm/memcontrol.c.  Later on it can be switched to use a separate config
option, so that the legacy code won't be compiled if not required.

Link: https://lkml.kernel.org/r/20240625005906.106920-1-roman.gushchin@linux.dev
Link: https://lkml.kernel.org/r/20240625005906.106920-2-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>