Re: [PATCH v2 hmm 02/11] mm/hmm: Use hmm_mirror not mm as an argument for hmm_range_register

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Tue, Jun 11, 2019 at 04:44:31PM -0300, Jason Gunthorpe wrote:
> On Sat, Jun 08, 2019 at 01:54:25AM -0700, Christoph Hellwig wrote:
> > FYI, I very much disagree with the direction this is moving.
> > 
> > struct hmm_mirror literally is a trivial duplication of the
> > mmu_notifiers.  All these drivers should just use the mmu_notifiers
> > directly for the mirroring part instead of building a thing wrapper
> > that adds nothing but helping to manage the lifetime of struct hmm,
> > which shouldn't exist to start with.
> 
> Christoph: What do you think about this sketch below?
> 
> It would replace the hmm_range/mirror/etc with a different way to
> build the same locking scheme using some optional helpers linked to
> the mmu notifier?
> 
> (just a sketch, still needs a lot more thinking)

I like the idea.  A few nitpicks:  Can we avoid having to store
the mm in struct mmu_notifier?  I think we could just easily pass
it as a parameter to the helpers.  The write lock case of
mm_invlock_start_write_and_lock is probably worth factoring into
separate helper? I can see cases where drivers want to just use
it directly if they need to force getting the lock without the chance
of a long wait.

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