Re: [PATCH V5 00/25] Live Update [restart] : fork mode?

[Steven Sistare]

On 7/30/2021 9:10 AM, Zheng Chuan wrote:
> Hi, Steve
> I have saw the discussion about the fork+exec mode below:
> https://www.mail-archive.com/qemu-devel@nongnu.org/msg815956.html
> 
> And I am still very curious and I want to discuss about the possibility to 
> support both fork+exec and exec in cpr framework.
> 
> 1.Why
> fork+exec could have some advantages and also drawbacks versus execvp() 
> directly.
> Advantages
> i) fork+exec give the chance to fallback to original process even after we do 
> exec which is important for workload seamless if any error happens.
> ii) smaller downtime since we could remove the vm_start() downtime out of the 
> frozen window.
> Drawbacks
> i）need more codes to handle including fork，address/ports conflict between 
> parent and child.
> ii）more complex life cycle management for the two processes.
> 
> 2.How
> The cpr framework is flexible and scalable, and maybe we can make use of most 
> codes to support both execvp and fork+exec mode.
> However, we may need to do more work compared to execvp method.
> i) do fork mode in a thread like migration thread
> ii) make parent and child talk to each other through socket or anonymous pipe
> iii）make use of sharing mechanism of fds in cpr framework including memfd, 
> vfio and devices fds
> iv）deal with the conflict about the socket address and port like vnc （do by 
> reuse port and pass the different args by cprexec）
> v) do life cycle managements for two qemu processes and need parent exit and 
> reconnection for the child at last by the management service
> 
> Please tell me if I am missing something important:)

Hi Zheng, that list sounds about right.  However, additional kernel changes 
would be needed to 
change ownership of the vfio device descriptors. The vfio module records the mm 
of the creating
process, and does not allow other processes to unmap ranges.  Also, the 
mm->locked_vm would 
need to be transferred to the new process.

Fork+exec could be a new mode to the cprsave command.

- Steve