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

[Jerome Glisse <jglisse@redhat.com>]

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:


Locking within the sync_cpu_device_pagetables() callback is the most important
aspect the driver must respect in order to keep things properly synchronized.
The usage pattern is::

 int driver_populate_range(...)
 {
      struct hmm_range range;
      ...

      range.start = ...;
      range.end = ...;
      range.pfns = ...;
      range.flags = ...;
      range.values = ...;
      range.pfn_shift = ...;
      hmm_range_register(&range);

      /*
       * Just wait for range to be valid, safe to ignore return value as we
       * will use the return value of hmm_range_snapshot() below under the
       * mmap_sem to ascertain the validity of the range.
       */
      hmm_range_wait_until_valid(&range, TIMEOUT_IN_MSEC);

 again:
      down_read(&mm->mmap_sem);
      ret = hmm_range_snapshot(&range);
      if (ret) {
          up_read(&mm->mmap_sem);
          if (ret == -EAGAIN) {
            /*
             * No need to check hmm_range_wait_until_valid() return value
             * on retry we will get proper error with hmm_range_snapshot()
             */
            hmm_range_wait_until_valid(&range, TIMEOUT_IN_MSEC);
            goto again;
          }
          hmm_range_unregister(&range);
          return ret;
      }
      take_lock(driver->update);
      if (!hmm_range_valid(&range)) {
          release_lock(driver->update);
          up_read(&mm->mmap_sem);
          goto again;
      }

      // Use pfns array content to update device page table

      hmm_range_unregister(&range);
      release_lock(driver->update);
      up_read(&mm->mmap_sem);
      return 0;
 }

The driver->update lock is the same lock that the driver takes inside its
sync_cpu_device_pagetables() callback. That lock must be held before calling
hmm_range_valid() to avoid any race with a concurrent CPU page table update.


Cheers,
Jérôme