Re: [PATCH v3 hmm 11/12] mm/hmm: Remove confusing comment and logic from hmm_release

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jun 19, 2019 at 12:53:55AM +0000, Kuehling, Felix wrote:
> This code is derived from our old MMU notifier code. Before HMM we used 
> to register a single MMU notifier per mm_struct and look up virtual 
> address ranges that had been registered for mirroring via driver API 
> calls. The idea was to reuse a single MMU notifier for the life time of 
> the process. It would remain registered until we got a notifier_release.
> 
> hmm_mirror took the place of that when we converted the code to HMM.
> 
> I suppose we could destroy the mirror earlier, when we have no more 
> registered virtual address ranges, and create a new one if needed later.

I didn't write the code, but if you look at hmm_mirror it already is
a multiplexer over the mmu notifier, and the intent clearly seems that
you register one per range that you want to mirror, and not multiplex
it once again.  In other words - I think each amdgpu_mn_node should
probably have its own hmm_mirror.  And while the amdgpu_mn_node objects
are currently stored in an interval tree it seems like they are only
linearly iterated anyway, so a list actually seems pretty suitable.  If
not we need to improve the core data structures instead of working
around them.