[ofiwg] fork support and MR cache

[Hefty, Sean]

> >>>>> There have been extensive discussions on github around the MR cache,
> >>>>> deadlocks, libibverbs madvise tracking, and fork.  The current
> >>>>> direction is to only enable the MR cache when fork is disabled.
> >>>>> This was done to work-around internal libibverbs tracking.  But I
> >>>>> suspect that bypassing that tracking (which is possible) can still
> >>>>> lead to issues when registrations are made through the MR cache.
> >>>>
> >>>> MADV_DONTFORK will be obsolete starting in kernel v5.9
> >>>>
> >>>> If you can test and confirm that everything works without it then we
> >>>> can detect and disable ibv_fork_init on new kernels.
> >>>
> >>> Interesting.  What will the behavior be for registered regions when fork is called?
> >>
> >> They are copied into the fork.
> >>
> >>> My concern is that the registrations are made and maintained without
> >>> the application being aware.  Will cached registrations need to be
> >>> released when fork is invoked, or is there some other mechanism
> >>> coming into play now?
> >>
> >> MRs continue to reliably point to memory owned in the parent
> >> process.
> >>
> >> The child process will be unable to use any MRs or verbs objects, just
> >> like today.
> >
> > Thanks - I think this means that fork becomes a non-issue.
> 
> We’ll have to figure out how to make some of these decisions at runtime.  We need a fix
> for today’s world (even if we’re ok with it being a little more hacky, since it has a
> finite lifespan), and a way to know whether we need to run that hacky fix or not in the
> future.  I don’t think our current hack around trying ibv_fork_init() will work in a
> world where rdma-core isn’t building the RB tree.  So some way of exposing that new
> behavior out of rdma-core would, unfortunately, be helpful to Libfabric.

I think a reasonable hack could be to implement the behavior from my first email.  That would involve bypassing the libibverbs calls and going directly to the verbs providers for reg/dereg mr.

- Sean