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

[Carlos Llamas <cmllamas@google.com>]

On Mon, Aug 10, 2026 at 11:54:04AM -0700, Suren Baghdasaryan wrote:
> On Mon, Aug 10, 2026 at 11:32 AM Carlos Llamas <cmllamas@google.com> wrote:
> >
> > On Thu, Aug 06, 2026 at 01:05:45PM -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.
> > > 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>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> > > Cc: Vlastimil Babka <vbabka@kernel.org>
> > > Cc: Shakeel Butt <shakeel.butt@linux.dev>
> > > Cc: linux-mm@kvack.org
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: Arve Hjønnevåg <arve@android.com>
> > > Cc: Todd Kjos <tkjos@android.com>
> > > Cc: Christian Brauner <christian@brauner.io>
> > > Cc: Carlos Llamas <cmllamas@google.com>
> > > Cc: Alice Ryhl <aliceryhl@google.com>
> > > Cc: "David S. Miller" <davem@davemloft.net>
> > > Cc: David Ahern <dsahern@kernel.org>
> > > Cc: netdev@vger.kernel.org
> > > ---
> > >  drivers/android/binder_alloc.c | 45 ++++++++++++++++------------------
> > >  1 file changed, 21 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > > index e4488ad86a65..c13a588c37de 100644
> > > --- a/drivers/android/binder_alloc.c
> > > +++ b/drivers/android/binder_alloc.c
> > > @@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> > >       struct vm_area_struct *vma;
> > >       struct page *page_to_free;
> > >       unsigned long page_addr;
> > > -     int mm_locked = 0;
> > >       size_t index;
> > >
> > >       if (!mmget_not_zero(mm))
> > > @@ -1151,27 +1150,25 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> > >       index = mdata->page_index;
> > >       page_addr = alloc->vm_start + index * PAGE_SIZE;
> > >
> > > -     /* attempt per-vma lock first */
> > > +     /*
> > > +      * Attempt per-vma lock. This is essentially a
> > > +      * "trylock". It can fail even if the VMA exists
> > > +      * for 'page_addr'.
> > > +      */
> >
> > Do we need to explain how lock_vma_under_rcu() works here?
> >
> > >       vma = lock_vma_under_rcu(mm, page_addr);
> > >       if (!vma) {
> > > -             /* fall back to mmap_lock */
> > > -             if (!mmap_read_trylock(mm))
> > > -                     goto err_mmap_read_lock_failed;
> > > -             mm_locked = 1;
> > > -             vma = vma_lookup(mm, page_addr);
> > > +             /*
> > > +              * If the vma exists, we can't continue because we cannot
> > > +              * remove the page from the vma. However, if the vma was
> > > +              * unmapped, it's okay to continue.
> > > +              */
> > > +             if (binder_alloc_is_mapped(alloc))
> > > +                     goto err_vma_lock_failed;
> >
> > The comments seem redundant, the label is enough. This works:
> >
> >         vma = lock_vma_under_rcu(mm, page_addr);
> >         if (!vma && binder_alloc_is_mapped(alloc))
> >                 goto err_vma_lock_failed;
> 
> Yeah, for the binder maintainers what's happening here is probably
> obvious, but when Alice explained the logic to me, this comment really
> clarified what's going on, so I added it here. If you insist on
> removing it, I'll do that of course.
> 
> >
> >
> > >       }
> > >
> > >       if (!mutex_trylock(&alloc->mutex))
> > >               goto err_get_alloc_mutex_failed;
> > >
> > > -     /*
> > > -      * Since a binder_alloc can only be mapped once, we ensure
> > > -      * the vma corresponds to this mapping by checking whether
> > > -      * the binder_alloc is still mapped.
> > > -      */
> > > -     if (vma && !binder_alloc_is_mapped(alloc))
> > > -             goto err_invalid_vma;
> > > -
> >
> > This introduces an "extra" change. We'll now release pages without a
> > valid vma (e.g. after munmap()). Before, these pages were expected to be
> > released via close() in binder_alloc_deferred_release() later.
> >
> > I don't see anything wrong with it. However, it does seem out of the
> > scope of this patch which just drops the mmap_read_lock() calls. Or at
> > least I don't see how this part is necessary.
> 
> This check came from this discussion:
> https://lore.kernel.org/all/anGrFIYiPhjxWSkD@google.com/
> It's added for consistency when handling cases where the original
> Binder VMA is gone.

Oh sorry I missed that thread. Yes, I agree we can reclaim these pages
earlier if really needed but my point is this is unrelated to the change
done here IMO. I couldn't figure out why that was needed.

If you are keeping that in the same commit it's probably worth adding an
explanation to the commit log?

--
Carlos Llamas