proc: add cond_resched to /proc/kpage* read/write loop

Reading/writing a /proc/kpage* file may take long on machines with a lot
of RAM installed.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Suggested-by: Andres Lagar-Cavilla <andreslc@google.com>
Reviewed-by: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Greg Thelen <gthelen@google.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/page.c

@@ -58,6 +58,8 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
 		pfn++;
 		out++;
 		count -= KPMSIZE;
+
+		cond_resched();
 	}
 
 	*ppos += (char __user *)out - buf;
@@ -219,6 +221,8 @@ static ssize_t kpageflags_read(struct file *file, char __user *buf,
 		pfn++;
 		out++;
 		count -= KPMSIZE;
+
+		cond_resched();
 	}
 
 	*ppos += (char __user *)out - buf;
@@ -267,6 +271,8 @@ static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
 		pfn++;
 		out++;
 		count -= KPMSIZE;
+
+		cond_resched();
 	}
 
 	*ppos += (char __user *)out - buf;