mm: memcontrol: introduce mem_cgroup_ino() and mem_cgroup_get_from_ino()

Patch series "mm: introduce shrinker debugfs interface", v5.

The only existing debugging mechanism is a couple of tracepoints in
do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end.  They
aren't covering everything though: shrinkers which report 0 objects will
never show up, there is no support for memcg-aware shrinkers.  Shrinkers
are identified by their scan function, which is not always enough (e.g. 
hard to guess which super block's shrinker it is having only
"super_cache_scan").

To provide a better visibility and debug options for memory shrinkers this
patchset introduces a /sys/kernel/debug/shrinker interface, to some extent
similar to /sys/kernel/slab.

For each shrinker registered in the system a directory is created.  As
now, the directory will contain only a "scan" file, which allows to get
the number of managed objects for each memory cgroup (for memcg-aware
shrinkers) and each numa node (for numa-aware shrinkers on a numa
machine).  Other interfaces might be added in the future.

To make debugging more pleasant, the patchset also names all shrinkers, so
that debugfs entries can have meaningful names.


This patch (of 5):

Shrinker debugfs requires a way to represent memory cgroups without using
full paths, both for displaying information and getting input from a user.

Cgroup inode number is a perfect way, already used by bpf.

This commit adds a couple of helper functions which will be used to handle
memcg-aware shrinkers.

Link: https://lkml.kernel.org/r/20220601032227.4076670-1-roman.gushchin@linux.dev
Link: https://lkml.kernel.org/r/20220601032227.4076670-2-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>