mm: shrinkers: add scan interface for shrinker debugfs

Add a scan interface which allows to trigger scanning of a particular
shrinker and specify memcg and numa node.  It's useful for testing,
debugging and profiling of a specific scan_objects() callback.  Unlike
alternatives (creating a real memory pressure and dropping caches via
/proc/sys/vm/drop_caches) this interface allows to interact with only one
shrinker at once.  Also, if a shrinker is misreporting the number of
objects (as some do), it doesn't affect scanning.

[roman.gushchin@linux.dev: improve typing, fix arg count checking]
Link: https://lkml.kernel.org/r/YpgKttTowT22mKPQ@carbon
[akpm@linux-foundation.org: fix arg count checking]
Link: https://lkml.kernel.org/r/20220601032227.4076670-7-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-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>

diff --git a/Documentation/admin-guide/mm/shrinker_debugfs.rst b/Documentation/admin-guide/mm/shrinker_debugfs.rst
index 1e0e5bd..3887f0b 100644 (file)
--- a/Documentation/admin-guide/mm/shrinker_debugfs.rst
+++ b/Documentation/admin-guide/mm/shrinker_debugfs.rst
@@ -5,14 +5,16 @@ Shrinker Debugfs Interface
 ==========================
 
 Shrinker debugfs interface provides a visibility into the kernel memory
-shrinkers subsystem and allows to get information about individual shrinkers.
+shrinkers subsystem and allows to get information about individual shrinkers
+and interact with them.
 
 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).
+Each shrinker directory contains **count** and **scan** files, which allow to
+trigger *count_objects()* and *scan_objects()* callbacks for each memcg and
+numa node (if applicable).
 
 Usage:
 ------
@@ -43,7 +45,7 @@ Usage:
 
     $ cd sb-btrfs\:vda2-24/
     $ ls
-    count
+    count            scan
 
 3. *Count objects*
 
@@ -102,3 +104,32 @@ Usage:
     2877 84 0
     293 1 0
     735 8 0
+
+4. *Scan objects*
+
+  The expected input format::
+
+    <cgroup inode id> <numa id> <number of objects to scan>
+
+  For a non-memcg-aware shrinker or on a system with no memory
+  cgrups **0** should be passed as cgroup id.
+  ::
+
+    $ cd /sys/kernel/debug/shrinker/
+    $ cd sb-btrfs\:vda2-24/
+
+    $ cat count | head -n 5
+    1 212 0
+    21 97 0
+    55 802 5
+    2367 2 0
+    225 13 0
+
+    $ echo "55 0 200" > scan
+
+    $ cat count | head -n 5
+    1 212 0
+    21 96 0
+    55 752 5
+    2367 2 0
+    225 13 0

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 781ecbd..e5b40c4 100644 (file)
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -99,6 +99,78 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
 }
 DEFINE_SHOW_ATTRIBUTE(shrinker_debugfs_count);
 
+static int shrinker_debugfs_scan_open(struct inode *inode, struct file *file)
+{
+       file->private_data = inode->i_private;
+       return nonseekable_open(inode, file);
+}
+
+static ssize_t shrinker_debugfs_scan_write(struct file *file,
+                                          const char __user *buf,
+                                          size_t size, loff_t *pos)
+{
+       struct shrinker *shrinker = file->private_data;
+       unsigned long nr_to_scan = 0, ino, read_len;
+       struct shrink_control sc = {
+               .gfp_mask = GFP_KERNEL,
+       };
+       struct mem_cgroup *memcg = NULL;
+       int nid;
+       char kbuf[72];
+       ssize_t ret;
+
+       read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
+       if (copy_from_user(kbuf, buf, read_len))
+               return -EFAULT;
+       kbuf[read_len] = '\0';
+
+       if (sscanf(kbuf, "%lu %d %lu", &ino, &nid, &nr_to_scan) != 3)
+               return -EINVAL;
+
+       if (nid < 0 || nid >= nr_node_ids)
+               return -EINVAL;
+
+       if (nr_to_scan == 0)
+               return size;
+
+       if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
+               memcg = mem_cgroup_get_from_ino(ino);
+               if (!memcg || IS_ERR(memcg))
+                       return -ENOENT;
+
+               if (!mem_cgroup_online(memcg)) {
+                       mem_cgroup_put(memcg);
+                       return -ENOENT;
+               }
+       } else if (ino != 0) {
+               return -EINVAL;
+       }
+
+       ret = down_read_killable(&shrinker_rwsem);
+       if (ret) {
+               mem_cgroup_put(memcg);
+               return ret;
+       }
+
+       sc.nid = nid;
+       sc.memcg = memcg;
+       sc.nr_to_scan = nr_to_scan;
+       sc.nr_scanned = nr_to_scan;
+
+       shrinker->scan_objects(shrinker, &sc);
+
+       up_read(&shrinker_rwsem);
+       mem_cgroup_put(memcg);
+
+       return size;
+}
+
+static const struct file_operations shrinker_debugfs_scan_fops = {
+       .owner   = THIS_MODULE,
+       .open    = shrinker_debugfs_scan_open,
+       .write   = shrinker_debugfs_scan_write,
+};
+
 int shrinker_debugfs_add(struct shrinker *shrinker)
 {
        struct dentry *entry;
@@ -128,6 +200,8 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
 
        debugfs_create_file("count", 0220, entry, shrinker,
                            &shrinker_debugfs_count_fops);
+       debugfs_create_file("scan", 0440, entry, shrinker,
+                           &shrinker_debugfs_scan_fops);
        return 0;
 }