[RFC][PATCH] Do not set /proc inode->pid for non-pid-related inodes

[Dave Hansen]

I was tracking down why we need find_get_pid(1) in
proc_get_sb(), when I realized that we apparently
don't need a pid at all in the non-pid parts of /proc.

Anyone see any problems with this approach?

----

For what I would imagine are historical reasons, we set
all struct proc_inode->pid fields.  We use the init
process for all non-/proc/<pid> inodes.

We get a handle to the init process in proc_get_sb()
then fetch it out in proc_pid_readdir(): 

	struct task_struct *reaper = get_proc_task(filp->f_path.dentry->d_inode);

The filp in that case is always the root inode on which
someone is doing a readdir.  This reaper variable gets
passed down into proc_base_instantiate() and eventually
set in the new inode's ->pid field.

The problem is that I don't see anywhere that we
actually go and use this, outside of the /proc/<pid>
directories.  Just referencing the init process like
this is a pain for containers because our init process
(pid == 1) can actually go away.

So, this patch removes all non-pid-dir use of
proc_inode->pid.  It puts a WARN_ON() in case anyone
tries to instantiate a proc inode with a pid in a place
we don't expect there to be one.

---

 lxc-dave/fs//proc/inode.c |    6 ------
 lxc-dave/fs/proc/base.c   |   31 ++++++++++---------------------
 2 files changed, 10 insertions(+), 27 deletions(-)

diff -puN fs//proc/inode.c~funny-proc-patch fs//proc/inode.c
--- lxc/fs//proc/inode.c~funny-proc-patch	2007-03-19 15:10:50.000000000 -0700
+++ lxc-dave/fs//proc/inode.c	2007-03-19 15:10:50.000000000 -0700
@@ -184,7 +184,6 @@ out_mod:
 int proc_fill_super(struct super_block *s, void *data, int silent)
 {
 	struct pid_namespace *pid_ns = data;
-	struct proc_inode *ei;
 	struct inode * root_inode;
 
 	s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
@@ -204,11 +203,6 @@ int proc_fill_super(struct super_block *
 	s->s_root = d_alloc_root(root_inode);
 	if (!s->s_root)
 		goto out_no_root;
-	/* Seed the root directory with a pid so it doesn't need
-	 * to be special in base.c.
-	 */
-	ei = PROC_I(root_inode);
-	ei->pid = find_get_pid(1);
 	return 0;
 
 out_no_root:
diff -puN fs//proc/internal.h~funny-proc-patch fs//proc/internal.h
diff -puN fs/proc/base.c~funny-proc-patch fs/proc/base.c
--- lxc/fs/proc/base.c~funny-proc-patch	2007-03-19 15:10:50.000000000 -0700
+++ lxc-dave/fs/proc/base.c	2007-03-19 15:11:40.000000000 -0700
@@ -1171,11 +1171,15 @@ static int pid_revalidate(struct dentry 
 
 static int pid_delete_dentry(struct dentry * dentry)
 {
+	struct pid *pid;
 	/* Is the task we represent dead?
 	 * If so, then don't put the dentry on the lru list,
 	 * kill it immediately.
 	 */
-	return !proc_pid(dentry->d_inode)->tasks[PIDTYPE_PID].first;
+	pid = proc_pid(dentry->d_inode);
+	if (!pid)
+		return 0;
+	return !pid->tasks[PIDTYPE_PID].first;
 }
 
 static struct dentry_operations pid_dentry_operations =
@@ -1813,6 +1817,7 @@ static struct dentry *proc_base_instanti
 	struct proc_inode *ei;
 	struct dentry *error = ERR_PTR(-EINVAL);
 
+	WARN_ON(task);
 	/* Allocate the inode */
 	error = ERR_PTR(-ENOMEM);
 	inode = new_inode(dir->i_sb);
@@ -1823,13 +1828,6 @@ static struct dentry *proc_base_instanti
 	ei = PROC_I(inode);
 	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 
-	/*
-	 * grab the reference to the task.
-	 */
-	ei->pid = get_task_pid(task, PIDTYPE_PID);
-	if (!ei->pid)
-		goto out_iput;
-
 	inode->i_uid = 0;
 	inode->i_gid = 0;
 	inode->i_mode = p->mode;
@@ -1847,9 +1845,6 @@ static struct dentry *proc_base_instanti
 	error = NULL;
 out:
 	return error;
-out_iput:
-	iput(inode);
-	goto out;
 }
 
 static struct dentry *proc_base_lookup(struct inode *dir, struct dentry *dentry)
@@ -1874,7 +1869,7 @@ static struct dentry *proc_base_lookup(s
 	if (p > last)
 		goto out;
 
-	error = proc_base_instantiate(dir, dentry, task, p);
+	error = proc_base_instantiate(dir, dentry, NULL, p);
 
 out:
 	put_task_struct(task);
@@ -1883,10 +1878,10 @@ out_no_task:
 }
 
 static int proc_base_fill_cache(struct file *filp, void *dirent, filldir_t filldir,
-	struct task_struct *task, struct pid_entry *p)
+	struct pid_entry *p)
 {
 	return proc_fill_cache(filp, dirent, filldir, p->name, p->len,
-				proc_base_instantiate, task, p);
+				proc_base_instantiate, NULL, p);
 }
 
 #ifdef CONFIG_TASK_IO_ACCOUNTING
@@ -2197,16 +2192,12 @@ static int proc_pid_fill_cache(struct fi
 int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
 {
 	unsigned int nr = filp->f_pos - FIRST_PROCESS_ENTRY;
-	struct task_struct *reaper = get_proc_task(filp->f_path.dentry->d_inode);
 	struct task_struct *task;
 	int tgid;
 
-	if (!reaper)
-		goto out_no_task;
-
 	for (; nr < ARRAY_SIZE(proc_base_stuff); filp->f_pos++, nr++) {
 		struct pid_entry *p = &proc_base_stuff[nr];
-		if (proc_base_fill_cache(filp, dirent, filldir, reaper, p) < 0)
+		if (proc_base_fill_cache(filp, dirent, filldir, p) < 0)
 			goto out;
 	}
 
@@ -2223,8 +2214,6 @@ int proc_pid_readdir(struct file * filp,
 	}
 	filp->f_pos = PID_MAX_LIMIT + TGID_OFFSET;
 out:
-	put_task_struct(reaper);
-out_no_task:
 	return 0;
 }
 
diff -puN fs/proc/root.c~funny-proc-patch fs/proc/root.c
_