mm: docs: document shrinker debugfs

Add a document describing the shrinker debugfs interface.

Link: https://lkml.kernel.org/r/20220601032227.4076670-5-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/index.rst

@@ -36,6 +36,7 @@ the Linux memory management.
    numa_memory_policy
    numaperf
    pagemap
+   shrinker_debugfs
    soft-dirty
    swap_numa
    transhuge

Documentation/admin-guide/mm/shrinker_debugfs.rst

@@ -0,0 +1,104 @@
+.. _shrinker_debugfs:
+
+==========================
+Shrinker Debugfs Interface
+==========================
+
+Shrinker debugfs interface provides a visibility into the kernel memory
+shrinkers subsystem and allows to get information about individual shrinkers.
+
+For each shrinker registered in the system a directory in **<debugfs>/shrinker/**
+is created. The directory's name is composed from the shrinker's name and an
+unique id: e.g. *kfree_rcu-0* or *sb-xfs:vda1-36*.
+
+Each shrinker directory contains the **count** file, which allows to trigger
+the *count_objects()* callback for each memcg and numa node (if applicable).
+
+Usage:
+------
+
+1. *List registered shrinkers*
+
+  ::
+
+    $ cd /sys/kernel/debug/shrinker/
+    $ ls
+    dquota-cache-16     sb-devpts-28     sb-proc-47       sb-tmpfs-42
+    mm-shadow-18        sb-devtmpfs-5    sb-proc-48       sb-tmpfs-43
+    mm-zspool:zram0-34  sb-hugetlbfs-17  sb-pstore-31     sb-tmpfs-44
+    rcu-kfree-0         sb-hugetlbfs-33  sb-rootfs-2      sb-tmpfs-49
+    sb-aio-20           sb-iomem-12      sb-securityfs-6  sb-tracefs-13
+    sb-anon_inodefs-15  sb-mqueue-21     sb-selinuxfs-22  sb-xfs:vda1-36
+    sb-bdev-3           sb-nsfs-4        sb-sockfs-8      sb-zsmalloc-19
+    sb-bpf-32           sb-pipefs-14     sb-sysfs-26      thp-deferred_split-10
+    sb-btrfs:vda2-24    sb-proc-25       sb-tmpfs-1       thp-zero-9
+    sb-cgroup2-30       sb-proc-39       sb-tmpfs-27      xfs-buf:vda1-37
+    sb-configfs-23      sb-proc-41       sb-tmpfs-29      xfs-inodegc:vda1-38
+    sb-dax-11           sb-proc-45       sb-tmpfs-35
+    sb-debugfs-7        sb-proc-46       sb-tmpfs-40
+
+2. *Get information about a specific shrinker*
+
+  ::
+
+    $ cd sb-btrfs\:vda2-24/
+    $ ls
+    count
+
+3. *Count objects*
+
+  Each line in the output has the following format::
+
+    <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1> ...
+    <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1> ...
+    ...
+
+  If there are no objects on all numa nodes, a line is omitted. If there
+  are no objects at all, the output might be empty.
+
+  If the shrinker is not memcg-aware or CONFIG_MEMCG is off, 0 is printed
+  as cgroup inode id. If the shrinker is not numa-aware, 0's are printed
+  for all nodes except the first one.
+  ::
+
+    $ cat count
+    1 224 2
+    21 98 0
+    55 818 10
+    2367 2 0
+    2401 30 0
+    225 13 0
+    599 35 0
+    939 124 0
+    1041 3 0
+    1075 1 0
+    1109 1 0
+    1279 60 0
+    1313 7 0
+    1347 39 0
+    1381 3 0
+    1449 14 0
+    1483 63 0
+    1517 53 0
+    1551 6 0
+    1585 1 0
+    1619 6 0
+    1653 40 0
+    1687 11 0
+    1721 8 0
+    1755 4 0
+    1789 52 0
+    1823 888 0
+    1857 1 0
+    1925 2 0
+    1959 32 0
+    2027 22 0
+    2061 9 0
+    2469 799 0
+    2537 861 0
+    2639 1 0
+    2707 70 0
+    2775 4 0
+    2877 84 0
+    293 1 0
+    735 8 0