[PATCH hmm 00/15] Consolidate the mmu notifier interval_tree and locking

[Jason Gunthorpe]

On Thu, Oct 17, 2019 at 04:47:20PM +0000, Koenig, Christian wrote:

> > get_user_pages/hmm_range_fault() and invalidate_range_start() both are
> > called while holding mm->map_sem, so they are always serialized.
> 
> Not even remotely.
> 
> For calling get_user_pages()/hmm_range_fault() you only need to hold the 
> mmap_sem in read mode.

Right
 
> And IIRC invalidate_range_start() is sometimes called without holding 
> the mmap_sem at all.

Yep
 
> So again how are they serialized?

The 'driver lock' thing does it, read the hmm documentation, the hmm
approach is basically the only approach that was correct of all the
drivers..

So long as the 'driver lock' is held the range cannot become
invalidated as the 'driver lock' prevents progress of invalidation.

Holding the driver lock and using the seq based mmu_range_read_retry()
tells if the previous unlocked get_user_pages() is still valid or
needs to be discard.

So it doesn't matter if get_user_pages() races or not, the result is not
to be used until the driver lock is held and mmu_range_read_retry()
called, which provides the coherence.

It is the usual seqlock pattern.

Jason