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

[Alice Ryhl]

On Wed, Feb 5, 2025 at 3:38 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Alice Ryhl <aliceryhl@xxxxxxxxxx> [250205 07:10]:
> > On Tue, Feb 4, 2025 at 4:46 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> 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.

Alice