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SubjectRe: pidns memory leak
Daniel Lezcano [dlezcano@fr.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@suse.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.

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

Sukadev


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