cgroup: introduce cgroup_ino()

mm/memory-failure.c::hwpoison_filter_task() has been reaching into
cgroup to extract the associated ino to be used as a filtering
criterion.  This is an implementation detail which shouldn't be
depended upon from outside cgroup proper and is about to change with
the scheduled kernfs conversion.

This patch introduces a proper interface to determine the associated
ino, cgroup_ino(), and updates hwpoison_filter_task() to use it
instead of reaching directly into cgroup.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>

include/linux/cgroup.h

@@ -507,6 +507,15 @@ static inline const char *cgroup_name(const struct cgroup *cgrp)
 	return rcu_dereference(cgrp->name)->name;
 }
 
+/* returns ino associated with a cgroup, 0 indicates unmounted root */
+static inline ino_t cgroup_ino(struct cgroup *cgrp)
+{
+	if (cgrp->dentry)
+		return cgrp->dentry->d_inode->i_ino;
+	else
+		return 0;
+}
+
 static inline struct cgroup_subsys_state *seq_css(struct seq_file *seq)
 {
 	struct cgroup_open_file *of = seq->private;

kernel/cgroup.c

@@ -792,7 +792,10 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
 	struct inode *inode = new_inode(sb);
 
 	if (inode) {
-		inode->i_ino = get_next_ino();
+		do {
+			/* ino 0 is reserved for dummy_root */
+			inode->i_ino = get_next_ino();
+		} while (!inode->i_ino);
 		inode->i_mode = mode;
 		inode->i_uid = current_fsuid();
 		inode->i_gid = current_fsgid();

mm/memory-failure.c

@@ -145,14 +145,10 @@ static int hwpoison_filter_task(struct page *p)
 		return -EINVAL;
 
 	css = mem_cgroup_css(mem);
-	/* root_mem_cgroup has NULL dentries */
-	if (!css->cgroup->dentry)
-		return -EINVAL;
-
-	ino = css->cgroup->dentry->d_inode->i_ino;
+	ino = cgroup_ino(css->cgroup);
 	css_put(css);
 
-	if (ino != hwpoison_filter_memcg)
+	if (!ino || ino != hwpoison_filter_memcg)
 		return -EINVAL;
 
 	return 0;