Re: [PATCH v13 2/8] mm: rust: add vm_area_struct methods that require read access

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Alice Ryhl <aliceryhl@google.com> [250205 10:24]:
> On Wed, Feb 5, 2025 at 3:38 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Alice Ryhl <aliceryhl@google.com> [250205 07:10]:
> > > On Tue, Feb 4, 2025 at 4:46 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > > > > > > > > +        let vma = unsafe { bindings::vma_lookup(self.mm.as_raw(), vma_addr) };
> > > > > > > > > +
> > > > > > > > > +        if vma.is_null() {
> > > > > > > > > +            None
> > > > > > > > > +        } else {
> > > > > > > > > +            // SAFETY: We just checked that a vma was found, so the pointer is valid. Furthermore,
> > > > > > > > > +            // the returned area will borrow from this read lock guard, so it can only be used
> > > > > > > > > +            // while the mmap read lock is still held.
> > > > > > > >
> > > > > > > > So We have complicated the locking of the vmas with rcu and per-vma
> > > > > > > > locking recently.  We are now able to look up and use a vma under the
> > > > > > > > rcu read lock.  Does this translate to rust model?
> > > > > > > >
> > > > > > > > I believe this is true in recent version of binder as well?
> > > > > > >
> > > > > > > Yes. The safety requirements of VmAreaRef is that you must hold the
> > > > > > > mmap read lock *or* the vma read lock while you have a VmAreaRef
> > > > > > > reference. This particular method achieves that requirement by holding
> > > > > > > the mmap read lock. But there is also a Rust lock_vma_under_rcu(), see
> > > > > > > patch 4 for that.
> > > > > >
> > > > > > Right, okay.  Thanks.  You can get the reference by only holding the rcu
> > > > > > read lock, but you should hold the vma lock to ensure that the vma
> > > > > > itself (and not just the pointer) is safe to use.
> > > > >
> > > > > Hmm... To modify the vma, you must hold the mmap *and* vma write lock,
> > > > > so holding the mmap read lock prevents mutations?
> > > >
> > > > Sorry, I think I confused things with my answer.  Your code is fine.
> > > > The phrasing of the "only be used while the mmap read lock is still
> > > > held" made me wonder about further clarification on the locking here
> > > > (because the locking is confusing).
> > > >
> > > > Yes, mmap read lock means there are no writers that can modify the vma.
> > > > Essentially, you are using the lock to ensure the entire vma space isn't
> > > > changed during your operation - which is heavier than just locking one
> > > > vma.
> > >
> > > I could extend the safety comment like this:
> > >
> > > SAFETY: We just checked that a vma was found, so the pointer is valid.
> > > Furthermore, the returned area will borrow from this read lock guard,
> > > so it can only be used while the mmap read lock is still held. This
> > > ensures that there are no writers because writers must hold both the
> > > mmap and vma write lock.
> >
> > How about just changing the last part to:
> >
> > Furthermore, the returned vma is still under the protection of the read
> > lock guard and can be used while the mmap read lock is still held.
> 
> Well, the important part here is that you can't do this:
> 
> let guard = mm.mmap_read_lock();
> let vma = guard.vma_lookup(...)?;
> drop(guard);
> vma.foo();
> 
> since that would use the vma after the lock has been released. The
> reason that the above is prevented is because `vma` borrows from
> `guard`, so you can only use `vma` while `guard` is still valid.
> 

But it implies that this isn't valid:

 let guard = mm.mmap_read_lock();
 let vma = guard.vma_lookup(...)?;

 vma_lock(vma);

 drop(guard);
 vma.foo();

 vma_unlock(vma);

See mm/userfaultfd.c:uffd_lock_vma(), which falls back to mmap read lock
to do this if rcu lock + lock_vma_under_rcu() is unable to lock the vma.

Thanks,
Liam