Re: [PATCH] common/mlx5: adjust fork call with the new kernel API

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 24 May 2023 15:01:40 +0300
<erezf@nvidia.com> wrote:

> From: Erez Ferber <erezf@nvidia.com>
> 
> While doing process fork() the operating system remaps all the parent
> process's memory to the address space of the child process and activates
> the Copy-on-Write mechanics - it duplicates physical pages once memory
> writing happens in the child process. Sometimes memory duplication is
> not allowed - for example, if the page contains hardware queue
> descriptors. To handle similar issues the rdma-core library should be
> prepared for forking.
> 
> The ibv_fork_init() prepares the library to track all the related memory
> and prevent it from forking using madvise() system API. This approach
> allows fork, but not all the memory is forked to the child process and,
> application should care not to touch pages where the parent application
> allocated the rdma-core objects.
> 
> The newer kernels propose an option of copy-on-fork for DMA pages and
> tracking all the memory and disabling it for the forking is no longer
> needed. The new API routine ibv_is_fork_initialized() should be involved
> to decide if library initialization for forking is required.
> 
> Fixes: 0e83b8e536 ("net/mlx5: move rdma-core calls to separate file")
> Cc: stable@dpdk.org
> Signed-off-by: Erez Ferber <erezf@nvidia.com>

I don't think DPDK applications should fork(), and lots other
parts of the shared huge pages will break if an application does this.