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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, May 24, 2019 at 12:49:02PM -0400, Jerome Glisse wrote:
> On Fri, May 24, 2019 at 11:36:49AM -0300, Jason Gunthorpe wrote:
> > On Thu, May 23, 2019 at 12:34:25PM -0300, Jason Gunthorpe wrote:
> > > From: Jason Gunthorpe <jgg@mellanox.com>
> > > 
> > > 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.
> 
> range->valid is use as bail early if invalidation is happening in
> hmm_range_fault() to avoid doing useless work. The synchronization
> is explained in the documentation:

That just says the hmm APIs handle locking. I asked how the apis
implement that locking internally.

Are you trying to say that if I do this, hmm will still work completely
correctly?

diff --git a/mm/hmm.c b/mm/hmm.c
index 8396a65710e304..42977744855d26 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -981,8 +981,8 @@ long hmm_range_snapshot(struct hmm_range *range)
 
 	do {
 		/* If range is no longer valid force retry. */
-		if (!range->valid)
-			return -EAGAIN;
+/*		if (!range->valid)
+			return -EAGAIN;*/
 
 		vma = find_vma(hmm->mm, start);
 		if (vma == NULL || (vma->vm_flags & device_vma))
@@ -1080,10 +1080,10 @@ long hmm_range_fault(struct hmm_range *range, bool block)
 
 	do {
 		/* If range is no longer valid force retry. */
-		if (!range->valid) {
+/*		if (!range->valid) {
 			up_read(&hmm->mm->mmap_sem);
 			return -EAGAIN;
-		}
+		}*/
 
 		vma = find_vma(hmm->mm, start);
 		if (vma == NULL || (vma->vm_flags & device_vma))
@@ -1134,7 +1134,7 @@ long hmm_range_fault(struct hmm_range *range, bool block)
 			start = hmm_vma_walk.last;
 
 			/* Keep trying while the range is valid. */
-		} while (ret == -EBUSY && range->valid);
+		} while (ret == -EBUSY /*&& range->valid*/);
 
 		if (ret) {
 			unsigned long i;