Re: pidns memory leak

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

Andrea,

We have been running a leak in child pid namespaces and some early debugging
points to the following commit:

>> 	commit 7766755a2f249e7e0dabc5255a0a3d151ff79821
>> 	Author: Andrea Arcangeli <andrea@suse.de>
>> 	Date:   Mon Feb 4 22:29:21 2008 -0800
>>

Reverting the commit seems to fix the leak but we need to do some more
analysis (like the lstat() question Daniel has).

However I have a basic question regarding the commit - the log mentions:

	> do_exit->release_task->mark_inode_dirty_sync->schedule() (will never
	> come back to run journal_stop)

But release_task() calls shrink_dcache_parent() for a _procfs_ dentry. Does
journal_stop() apply to procfs also ?

Thanks,

Sukadev


Daniel Lezcano [dlezcano@fr.ibm.com] wrote:
> Sukadev Bhattiprolu wrote:
>> 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.
>
> 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

| #include <stdio.h>
| #include <unistd.h>
| #include <stdlib.h>
| #include <sys/prctl.h>
| #include <sys/param.h>
| #include <sys/stat.h>
| #include <sys/poll.h>
| #include <signal.h>
| #include <sched.h>
| 
| #ifndef CLONE_NEWPID
| #  define CLONE_NEWPID            0x20000000
| #endif
| 
| int child(void *arg)
| {
| 	pid_t pid;
| 	struct stat s;
| 
| 	if (mount("proc", "/proc", "proc", 0, NULL)) {
| 		perror("mount");
| 		return -1;
| 	}
| 
| 	pid = fork();
| 	if (pid < 0) {
| 		perror("fork");
| 		return -1;
| 	}
| 
| 	if (!pid) {
| 		poll(0, 0 , -1);
| 		exit(-1);
| 	}
| 
| 	poll(0, 0, -1);
| 
| 	return 0;
| }
| 
| pid_t clonens(int (*fn)(void *), void *arg, int flags)
| {
| 	long stack_size = sysconf(_SC_PAGESIZE);
|  	void *stack = alloca(stack_size) + stack_size;
| 	return clone(fn, stack, flags | SIGCHLD, arg);
| }
| 
| int main(int argc, char *argv[])
| {
| 	pid_t pid;
| 	struct stat s;
| 	char path[MAXPATHLEN];
| 
| 	pid = clonens(child, NULL, CLONE_NEWNS|CLONE_NEWPID);
| 	if (pid < 0) {
| 		perror("clone");
| 		return -1;
| 	}
| 
| 	/* yes ugly.*/
| 	sleep(1);
| 
| 	/* !! assumption : child of my child is pid + 1 
| 	 * any reliable simple solution is welcome :) */
| 	snprintf(path, sizeof(path), "/proc/%d/exe", pid + 1);
| 	
| 	if (lstat(path, &s)) {
| 		perror("lstat");
| 		exit(-1);
| 	}
| 
| 	if (kill(pid, SIGKILL)) {
| 		perror("kill");
| 		return -1;
| 	}
| 
| 	return 0;
| }