linux-kernel - Re: pidns memory leak

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Message-ID: <4ACF381F.9050808@fr.ibm.com>
Date:	Fri, 09 Oct 2009 15:18:23 +0200
From:	Daniel Lezcano <dlezcano@...ibm.com>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
CC:	Pavel Emelianov <xemul@...nvz.org>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: pidns memory leak

Sukadev Bhattiprolu wrote:
> Daniel Lezcano [dlezcano@...ibm.com] wrote:
>> Sukadev Bhattiprolu wrote:
>>> Still digging through some traces, but below I have some questions that 
>>> I am still trying to answer.
>>>
>>>> I am not sure what you mean by 'struct pids' but what I observed is:
>>> Ok, I see that too. If pids leak, then pid-namespace will leak too.
>>> Do you see any leaks in proc_inode_cache ?
>> Yes, right. It leaks too.
> 
> Ok, some progress...
> 
> Can you please verify these observations:
> 
> - If the container exits normally, the leak does not seem to happen.
>   (i.e reduce your sleep 3600 to say sleep 3 and remove the lxc-stop).
> 
> - Revert the following commit and check if the leak happens:
> 
> 	commit 7766755a2f249e7e0dabc5255a0a3d151ff79821
> 	Author: Andrea Arcangeli <andrea@...e.de>
> 	Date:   Mon Feb 4 22:29:21 2008 -0800
> 
> (this commit added the check for PF_EXITING in proc_flush_task_mnt 
> loosely explained below).



> Incomplete analysis :-)
> 
> If the container-init is terminated (by the lxc-stop), the container zaps
> other processes in the container and waits for them. The leak happens in
> this case.
> 
> Following sequence of events occur:
> 
> 	- container-init calls do_exit and sets PF_EXITING (in exit_signals())
> 
> 	- container init calls zaps_pid_ns_processes() (exit_notify /
> 	  forget_orignal_parent() / find_new_reaper())
> 
> 	- In zap_pid_ns_processes() container-init sends SIGKILL to
> 	  descendants and calls sys_wait().
> 
> 	- The sys_wait() is expected to call release_task() which calls
> 	  proc_flush_task_mnt().
> 
> 	- proc_flush_task_mnt() looks up the dentry for the pid (2 in
> 	  our example) and finds the dentry.
> 
> 	  But since container-init is itself exiting (i.e PF_EXITING is
> 	  set) it does NOT call the shrink_dcache_parent(), but,
> 	  interestingly calls d_drop() and dput().
> 
> 	  Now the d_drop() unhashes the dentry for the pid 2.
> 
> 	- proc_flush_task_mnt() then tries to find the dentry for the
> 	  tgid of the process. In our case, the tgid == pid == 2 and
> 	  we just unhashed the dentry for "2".
> 
> 	  So, we don't find the dentry for the leader either (and hence
> 	  don't make the second shrink_dcache_parent() call in
> 	  proc_flush_task_mnt() either).
> 
> 	  Without a call to shrink_dcache_parent(), the proc inode
> 	  for the process that was terminated by container init is
> 	  not deleted (i.e we don't call proc_delete_inode() or
> 	  the put_pid() inside it) causing us to leak proc_inodes,
> 	  struct pid and hence struct pid_namespace.

Ouch !

Nice analysis :)

Following your explanation I was able to reproduce a simple program 
added in attachment. But there is something I do not understand is why 
the leak does not appear if I do the 'lstat' (cf. test program) in the 
pid 2 context.


> There should be a better fix, but first please confirm if reverting the
> above commit fixes the leak for you also.

I confirm the leak does no longer appear when reverting this patch.

Thanks
   -- Daniel

View attachment "bugpidns_leak.c" of type "text/x-csrc" (1260 bytes)