Re: [PATCH v4 2/5] binder: Make shrinker rely solely on per-VMA lock

On Thu, Aug 6, 2026 at 10:05 PM Suren Baghdasaryan [off-list ref] 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.
The removal of the fallback does not affect NOMMU case because binder
driver depends on CONFIG_MMU.

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>