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

[Christoph Hellwig]

On Wed, Jun 12, 2019 at 08:41:25AM -0300, Jason Gunthorpe wrote:
> > 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.
> 
> Yes, but I think any driver that needs to use this API will have to
> hold the 'struct mm_struct' and the 'struct mmu_notifier' together (ie
> ODP does this in ib_ucontext_per_mm), so if we put it in the notifier
> then it is trivially available everwhere it is needed, and the
> mmu_notifier code takes care of the lifetime for the driver.

True.  Well, maybe keep it for now at least.

> The entire purpose of the invlock is to avoid getting the write lock
> on mmap_sem as a fast path - if the driver wishes to use mmap_sem
> locking only then it should just do so directly and forget about the
> invlock.

May worry here is that there migh be cases where the driver needs
to expedite the action, in which case jumping straight to the write
lock.  But again we can probably skip this for now and see if it really
ends up being needed.