[PATCH] proc: don't lock task_structs indefinitely

Every inode in /proc holds a reference to a struct task_struct.  If a
directory or file is opened and remains open after the the task exits this
pinning continues.  With 8K stacks on a 32bit machine the amount pinned per
file descriptor is about 10K.

Normally I would figure a reasonable per user process limit is about 100
processes.  With 80 processes, with a 1000 file descriptors each I can trigger
the 00M killer on a 32bit kernel, because I have pinned about 800MB of useless
data.

This patch replaces the struct task_struct pointer with a pointer to a struct
task_ref which has a struct task_struct pointer.  The so the pinning of dead
tasks does not happen.

The code now has to contend with the fact that the task may now exit at any
time.  Which is a little but not muh more complicated.

With this change it takes about 1000 processes each opening up 1000 file
descriptors before I can trigger the OOM killer.  Much better.

[mlp@google.com: task_mmu small fixes]
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Paul Jackson <pj@sgi.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Albert Cahalan <acahalan@gmail.com>
Signed-off-by: Prasanna Meda <mlp@google.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 20 insertions, 7 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index b602f73fb38d..3e991c0c02e2 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -50,6 +50,7 @@
 #include <linux/time.h>
 #include <linux/backing-dev.h>
 #include <linux/sort.h>
+#include <linux/task_ref.h>
 
 #include <asm/uaccess.h>
 #include <asm/atomic.h>
@@ -2442,31 +2443,43 @@ void __cpuset_memory_pressure_bump(void)
  */
 static int proc_cpuset_show(struct seq_file *m, void *v)
 {
+	struct task_ref *tref;
 	struct task_struct *tsk;
 	char *buf;
-	int retval = 0;
+	int retval;
 
+	retval = -ENOMEM;
 	buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
 	if (!buf)
-		return -ENOMEM;
+		goto out;
+
+	retval = -ESRCH;
+	tref = m->private;
+	tsk = get_tref_task(tref);
+	if (!tsk)
+		goto out_free;
 
-	tsk = m->private;
+	retval = -EINVAL;
 	mutex_lock(&manage_mutex);
+
 	retval = cpuset_path(tsk->cpuset, buf, PAGE_SIZE);
 	if (retval < 0)
-		goto out;
+		goto out_unlock;
 	seq_puts(m, buf);
 	seq_putc(m, '\n');
-out:
+out_unlock:
 	mutex_unlock(&manage_mutex);
+	put_task_struct(tsk);
+out_free:
 	kfree(buf);
+out:
 	return retval;
 }
 
 static int cpuset_open(struct inode *inode, struct file *file)
 {
-	struct task_struct *tsk = PROC_I(inode)->task;
-	return single_open(file, proc_cpuset_show, tsk);
+	struct task_ref *tref = PROC_I(inode)->tref;
+	return single_open(file, proc_cpuset_show, tref);
 }
 
 struct file_operations proc_cpuset_operations = {