Re: [RFC PATCH 00/11] mm/hmm: Various revisions from a locking/code review

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On Thu, May 23, 2019 at 12:34:25PM -0300, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> 
> This patch series arised out of discussions with Jerome when looking at the
> ODP changes, particularly informed by use after free races we have already
> found and fixed in the ODP code (thanks to syzkaller) working with mmu
> notifiers, and the discussion with Ralph on how to resolve the lifetime model.

So the last big difference with ODP's flow is how 'range->valid'
works.

In ODP this was done using the rwsem umem->umem_rwsem which is
obtained for read in invalidate_start and released in invalidate_end.

Then any other threads that wish to only work on a umem which is not
undergoing invalidation will obtain the write side of the lock, and
within that lock's critical section the virtual address range is known
to not be invalidating.

I cannot understand how hmm gets to the same approach. It has
range->valid, but it is not locked by anything that I can see, so when
we test it in places like hmm_range_fault it seems useless..

Jerome, how does this work?

I have a feeling we should copy the approach from ODP and use an
actual lock here.

Jason




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