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

[Pavel Emelianov]

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.

>  	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.

>  	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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