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

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 25 May 2023 08:10:03 +0000
Slava Ovsiienko <viacheslavo@nvidia.com> wrote:

> > -----Original Message-----
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > Sent: Wednesday, May 24, 2023 5:50 PM
> > To: Erez Ferber <erezf@nvidia.com>
> > Cc: dev@dpdk.org; Slava Ovsiienko <viacheslavo@nvidia.com>; Matan Azrad
> > <matan@nvidia.com>; Raslan Darawsheh <rasland@nvidia.com>;
> > stable@dpdk.org
> > Subject: Re: [PATCH] common/mlx5: adjust fork call with the new kernel API
> > 
> > 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>  
> >   
> Hi,
> 
> > I don't think DPDK applications should fork(), and lots other parts of the
> > shared huge pages will break if an application does this.  
> 
> I agree - application should not, we have the secondary/primary processes approach.
> Nonetheless, we have the real use case - DPDK application does fork() and works well.
> Without mlx5 PMD 😊. With mlx5 it ran into some troubles. Now we have the solution.

The problem is you are allowing fork(). And many other libraries may break.
Imagine a DPDK library which has some local mutex and hugepages.
If two forked processes use it then the locks won't work and hugepage data
will be corrupted.