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@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.
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
