kernel: cgroup: add poll file operation

Cgroup has a standardized poll/notification mechanism for waking all
pollers on all fds when a filesystem node changes.  To allow polling for
custom events, add a .poll callback that can override the default.

This is in preparation for pollable cgroup pressure files which have
per-fd trigger configurations.

Link: http://lkml.kernel.org/r/20190124211518.244221-3-surenb@google.comSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cgroup-defs.h

@@ -32,6 +32,7 @@ struct kernfs_node;
 struct kernfs_ops;
 struct kernfs_open_file;
 struct seq_file;
+struct poll_table_struct;
 
 #define MAX_CGROUP_TYPE_NAMELEN 32
 #define MAX_CGROUP_ROOT_NAMELEN 64
@@ -574,6 +575,9 @@ struct cftype {
 	ssize_t (*write)(struct kernfs_open_file *of,
 			 char *buf, size_t nbytes, loff_t off);
 
+	__poll_t (*poll)(struct kernfs_open_file *of,
+			 struct poll_table_struct *pt);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lock_class_key	lockdep_key;
 #endif

kernel/cgroup/cgroup.c

@@ -3534,6 +3534,16 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 	return ret ?: nbytes;
 }
 
+static __poll_t cgroup_file_poll(struct kernfs_open_file *of, poll_table *pt)
+{
+	struct cftype *cft = of->kn->priv;
+
+	if (cft->poll)
+		return cft->poll(of, pt);
+
+	return kernfs_generic_poll(of, pt);
+}
+
 static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
 {
 	return seq_cft(seq)->seq_start(seq, ppos);
@@ -3572,6 +3582,7 @@ static struct kernfs_ops cgroup_kf_single_ops = {
 	.open			= cgroup_file_open,
 	.release		= cgroup_file_release,
 	.write			= cgroup_file_write,
+	.poll			= cgroup_file_poll,
 	.seq_show		= cgroup_seqfile_show,
 };
 
@@ -3580,6 +3591,7 @@ static struct kernfs_ops cgroup_kf_ops = {
 	.open			= cgroup_file_open,
 	.release		= cgroup_file_release,
 	.write			= cgroup_file_write,
+	.poll			= cgroup_file_poll,
 	.seq_start		= cgroup_seqfile_start,
 	.seq_next		= cgroup_seqfile_next,
 	.seq_stop		= cgroup_seqfile_stop,