Re: [PATCH v3 2/5] binder: Make shrinker rely solely on per-VMA lock

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Aug 03, 2026 at 09:48:19AM +0000, Alice Ryhl wrote:
> On Sun, Aug 02, 2026 at 02:54:56PM -0700, Suren Baghdasaryan wrote:
> > From: Dave Hansen <dave.hansen@linux.intel.com>
> >
> > tl;dr: lock_vma_under_rcu() is already a trylock. No need to do both
> > it and mmap_read_trylock().
> >
> > Long Version:
> >
> > == Background ==
> >
> > Historically, binder used an mmap_read_trylock() in its shrinker code.
> > This ensures that reclaim is not blocked on an mmap_lock. Commit
> > 95bc2d4a9020 ("binder: use per-vma lock in page reclaiming") added
> > support for the per-VMA lock, but left mmap_read_trylock() as a
> > fallback.
> >
> > This was presumably because the per-VMA locking can fail for several
> > reasons and most (all?) lock_vma_under_rcu() callers have a fallback
> > to mmap_read_trylock().
> >
> > == Problem ==
> >
> > The fallback is not worth the complexity here. lock_vma_under_rcu() is
> > essentially already a non-blocking trylock. The main reason it fails
> > is also the reason mmap_read_trylock() fails: something is holding
> > mmap_write_lock().
> >
> > The only remedy for a collision with mmap_write_lock() is to wait,
> > which this code can not do. So the "fallback" after
> > lock_vma_under_rcu() failure is not really a fallback: it is really
> > likely to just be retrying in vain. That retry in an of itself isn't
> > horrible. But it adds complexity.
> >
> > == Solution ==
> >
> > Now that per-VMA locks are universally available, lock_vma_under_rcu()
> > will not persistently fail. Rely on it alone and simplify the code.
> >
> > Full disclosure: I originally tried to do this with
> > lock_vma_under_rcu_wait(), but it did not fit well with the mmap_lock
> > trylock semantics. Claude caught this in a review and suggested the
> > approach in this path. It seemed sane to me. So, Suggesed-by: Claude,
> > I guess.
> >
> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Is there anything on the rust side that needs to be changed also?

--
Cheers, Lorenzo