Re: [PATCH 1/1] RDMA/odp: convert to use HMM for ODP

[Jason Gunthorpe]

On Wed, Feb 20, 2019 at 05:29:24PM -0500, Jerome Glisse wrote:
> > > 
> > > Yes it is safe, the hmm struct has its own refcount and mirror holds a
> > > reference on it, the mm struct itself has a reference on the mm
> > > struct.
> > 
> > The issue here is that that hmm_mirror_unregister() must be a strong
> > fence that guarentees no callback is running or will run after
> > return. mmu_notifier_unregister did not provide that.
> > 
> > I think I saw locking in hmm that was doing this..
> 
> So pattern is:
>     hmm_mirror_register(mirror);
> 
>     // Safe for driver to call within HMM with mirror no matter what
> 
>     hmm_mirror_unregister(mirror)
> 
>     // Driver must no stop calling within HMM, it would be a use after
>     // free scenario

This statement is the opposite direction

I want to know that HMM doesn't allow any driver callbacks to be
running after unregister - because I am going to kfree mirror and
other memory touched by the driver callbacks.

Jason