/proc/swaps: support polling

System management wants to subscribe to changes in swap configuration.
Make /proc/swaps pollable like /proc/mounts.

[akpm@linux-foundation.org: document proc_poll_event]
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Acked-by: Greg KH <greg@kroah.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -30,6 +30,7 @@
 #include <linux/capability.h>
 #include <linux/syscalls.h>
 #include <linux/memcontrol.h>
+#include <linux/poll.h>
 
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
@@ -58,6 +59,10 @@ static struct swap_info_struct *swap_info[MAX_SWAPFILES];
 
 static DEFINE_MUTEX(swapon_mutex);
 
+static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
+/* Activity counter to indicate that a swapon or swapoff has occurred */
+static atomic_t proc_poll_event = ATOMIC_INIT(0);
+
 static inline unsigned char swap_count(unsigned char ent)
 {
 	return ent & ~SWAP_HAS_CACHE;	/* may include SWAP_HAS_CONT flag */
@@ -1680,6 +1685,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	}
 	filp_close(swap_file, NULL);
 	err = 0;
+	atomic_inc(&proc_poll_event);
+	wake_up_interruptible(&proc_poll_wait);
 
 out_dput:
 	filp_close(victim, NULL);
@@ -1688,6 +1695,25 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 }
 
 #ifdef CONFIG_PROC_FS
+struct proc_swaps {
+	struct seq_file seq;
+	int event;
+};
+
+static unsigned swaps_poll(struct file *file, poll_table *wait)
+{
+	struct proc_swaps *s = file->private_data;
+
+	poll_wait(file, &proc_poll_wait, wait);
+
+	if (s->event != atomic_read(&proc_poll_event)) {
+		s->event = atomic_read(&proc_poll_event);
+		return POLLIN | POLLRDNORM | POLLERR | POLLPRI;
+	}
+
+	return POLLIN | POLLRDNORM;
+}
+
 /* iterator */
 static void *swap_start(struct seq_file *swap, loff_t *pos)
 {
@@ -1771,14 +1797,32 @@ static const struct seq_operations swaps_op = {
 
 static int swaps_open(struct inode *inode, struct file *file)
 {
-	return seq_open(file, &swaps_op);
+	struct proc_swaps *s;
+	int ret;
+
+	s = kmalloc(sizeof(struct proc_swaps), GFP_KERNEL);
+	if (!s)
+		return -ENOMEM;
+
+	file->private_data = s;
+
+	ret = seq_open(file, &swaps_op);
+	if (ret) {
+		kfree(s);
+		return ret;
+	}
+
+	s->seq.private = s;
+	s->event = atomic_read(&proc_poll_event);
+	return ret;
 }
 
 static const struct file_operations proc_swaps_operations = {
 	.open		= swaps_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
 	.release	= seq_release,
+	.poll		= swaps_poll,
 };
 
 static int __init procswaps_init(void)
@@ -2084,6 +2128,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		swap_info[prev]->next = type;
 	spin_unlock(&swap_lock);
 	mutex_unlock(&swapon_mutex);
+	atomic_inc(&proc_poll_event);
+	wake_up_interruptible(&proc_poll_wait);
+
 	error = 0;
 	goto out;
 bad_swap: