[Devel] [PATCH 17/17] Pid-NS(V3) Introduce proc_mnt for pid_ns

[sukadev at us.ibm.com]

Pavel Emelianov [xemul at openvz.org] wrote:
| sukadev at us.ibm.com wrote:
| > From: sukadev at linux.vnet.ibm.com
| > Subject: [PATCH 17/17] Pid-NS(V3) Introduce proc_mnt for pid_ns
| > 
| > The following patch completes the removal of the global proc_mnt.
| > It fetches the mnt on which to do dentry invalidations from the
| > pid_namespace in which the task appears.
| > 
| > For now, there is only one pid namespace in mainline so this is
| > straightforward.  In the -lxc tree we'll have to do something
| > more complex.  The proc_flush_task() code takes a task, and
| > needs to be able to find the corresponding proc superblocks on
| > which that tasks's /proc/<pid> directories could appear.  We
| > can tell in which pid namespaces a task appears, so I put a
| > pointer from the pid namespace to the corresponding proc_mnt.
| > 
| > /proc currently has some special code to make sure that the root
| > directory gets set up correctly.  It proc_mnt variable in order
| > to find its way to the root inode.
| > 
| > Changelog:
| > 
| > 	2.6.22-rc4-mm2-pidns1:
| > 
| > 	- Call proc_fill_super once per pid namespace
| > 	- Call proc_flush_task() before detaching a task's 'struct pid'.
| > 	- Get a reference to pid namespace when mounting/remounting /proc.
| > 	  Put this reference when unmounting.
| > 
| > Signed-off-by: Dave Hansen <haveblue at us.ibm.com>
| > Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
| > ---
| > 
| >  fs/proc/base.c                |   30 ++++++++++++++++++-
| >  fs/proc/inode.c               |   12 ++++++-
| >  fs/proc/root.c                |   65 ++++++++++++++++++++++++++++++------------
| >  include/linux/pid_namespace.h |    1 
| >  include/linux/proc_fs.h       |    1 
| >  kernel/exit.c                 |    2 -
| >  6 files changed, 87 insertions(+), 24 deletions(-)
| > 
| [snip]
| 
| > @@ -55,12 +90,6 @@ void __init proc_root_init(void)
| >  	err = register_filesystem(&proc_fs_type);
| >  	if (err)
| >  		return;
| > -	proc_mnt = kern_mount(&proc_fs_type);
| > -	err = PTR_ERR(proc_mnt);
| > -	if (IS_ERR(proc_mnt)) {
| > -		unregister_filesystem(&proc_fs_type);
| > -		return;
| > -	}
| 
| Wow! Is this safe? Everyone expexts proc_mnt to be not NULL.

Can you elaborate ? Between the previous patch (i.e 16/17) and this
we made sure no one is using the global proc_mnt. Everyone should be
using the pid_ns->proc_mnt instead.

| 
| >  	proc_misc_init();
| >  	proc_net = proc_mkdir("net", NULL);
| >  	proc_net_stat = proc_mkdir("net/stat", NULL);
| 
| [snip]
| 
| > Index: lx26-22-rc4-mm2/kernel/exit.c
| > ===================================================================
| > --- lx26-22-rc4-mm2.orig/kernel/exit.c	2007-06-16 04:15:23.000000000 -0700
| > +++ lx26-22-rc4-mm2/kernel/exit.c	2007-06-16 04:15:23.000000000 -0700
| > @@ -157,6 +157,7 @@ void release_task(struct task_struct * p
| >  	struct task_struct *leader;
| >  	int zap_leader;
| >  repeat:
| > +	proc_flush_task(p);
| 
| Such a flush won't actually remove *any* dentries from the tree and thus
| may be omitted.

Again, can you elaborate ?

The previous version had proc_flush_task() much later in release_task() and
that did not free dentries bc task_pid() was NULL. Moving proc_flush_task()
up, should fix that and it did fix the leak we were seeing before.

| 
| >  	atomic_dec(&p->user->processes);
| >  	write_lock_irq(&tasklist_lock);
| >  	ptrace_unlink(p);
| > @@ -185,7 +186,6 @@ repeat:
| >  	}
| >  
| >  	write_unlock_irq(&tasklist_lock);
| > -	proc_flush_task(p);
| >  	release_thread(p);
| >  	call_rcu(&p->rcu, delayed_put_task_struct);
| >  
| > 
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