[pve-devel] PVE child process behavior question

[Denis Kanchev]

The parent of the storage migration process gets killed.

It seems that this is the desired behavior and as far i understand it 
correctly - the child worker is detached from the parent and it has 
nothing to do with it after spawning.

Thanks for the information, it was very helpful.

On 22.05.25 г. 9:30 ч., Fabian Grünbichler wrote:
>> Denis Kanchev via pve-devel <pve-devel at lists.proxmox.com> hat am 21.05.2025 15:13 CEST geschrieben:
>> Hello,
>>
>> We had an issue with a customer migrating a VM between nodes using our
>> shared storage solution.
>>
>> On the target host the OOM killer killed the main migration process, but
>> the child process (which actually performs the migration) kept on
>> working, which we did not expect, and that caused some issues.
> could you be more specific which process got killed?
>
> when you do a migration, a task worker is forked and its UPID is returned
> to the caller for further querying.
>
> as part of the migration, other processes get spawned:
> - ssh tunnel to the target node
> - storage migration processes (on both nodes)
> - VM state management CLI calls (on the target node)
>
> which of those is the "main migration process"? which is the child process?
>
>> This leads us to the broader question - after a request is submitted,
>> the parent can be terminated, and not return a response to the client,
>> while the work is being done, and the request can be wrongly retried or
>> considered unfinished.
> the parent should return almost immediately, as all it is doing at that
> point is returning the UPID to the client (the process then continues to
> do other work though, but that is no longer related to this task).
>
> the only exception is for "sync" task workers, like in a CLI context,
> where the "parent" has no other work to do, so it waits for the child/task
> to finish and prints its output while doing so, and some "bulk action"
> style API calls that fork multiple task workers and poll them themselves.
>   
>> Should the child processes terminate together with the parent to guard
>> against this, or is this expected behavior?
> the parent (API worker process) and child (task worker process) have no
> direct relation after the task worker has been spawned.
>
>> Here is an example patch to do this:
>>
>>
>> diff --git a/src/PVE/RESTEnvironment.pm b/src/PVE/RESTEnvironment.pm
>>
>> index bfde7e6..744fffc 100644
>>
>> --- a/src/PVE/RESTEnvironment.pm
>>
>> +++ b/src/PVE/RESTEnvironment.pm
>>
>> @@ -13,8 +13,9 @@ use Fcntl qw(:flock);
>>
>>    use IO::File;
>>
>>    use IO::Handle;
>>
>>    use IO::Select;
>>
>> -use POSIX qw(:sys_wait_h EINTR);
>>
>> +use POSIX qw(:sys_wait_h EINTR SIGKILL);
>>
>>    use AnyEvent;
>>
>> +use Linux::Prctl qw(set_pdeathsig);
>>
>>
>>    use PVE::Exception qw(raise raise_perm_exc);
>>
>>    use PVE::INotify;
>>
>> @@ -549,6 +550,9 @@ sub fork_worker {
>>
>> POSIX::setsid();
>>
>>       }
>>
>>
>> +   # The signal that the calling process will get when its parent dies
>>
>> +   set_pdeathsig(SIGKILL);
> that has weird implications with regards to threads, so I don't think that
> is a good idea..
>
>> +
>>
>> POSIX::close ($psync[0]);
>>
>> POSIX::close ($ctrlfd[0]) if $sync;
>>
>> POSIX::close ($csync[1]);