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

[Suren Baghdasaryan]

On Mon, Aug 10, 2026 at 11:32 AM Carlos Llamas <cmllamas@xxxxxxxxxx> wrote:
>
> On Thu, Aug 06, 2026 at 01:05:45PM -0700, Suren Baghdasaryan wrote:
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> > Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Arve Hjønnevåg <arve@xxxxxxxxxxx>
> > Cc: Todd Kjos <tkjos@xxxxxxxxxxx>
> > Cc: Christian Brauner <christian@xxxxxxxxxx>
> > Cc: Carlos Llamas <cmllamas@xxxxxxxxxx>
> > Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > Cc: David Ahern <dsahern@xxxxxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx
> > ---
> >  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@xxxxxxxxxx/
It's added for consistency when handling cases where the original
Binder VMA is gone.

>
> >       trace_binder_unmap_kernel_start(alloc, index);
> >
> >       page_to_free = alloc->pages[index];
> > @@ -1182,7 +1179,12 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> >       list_lru_isolate(lru, item);
> >       spin_unlock(&lru->lock);
> >
> > -     if (vma) {
> > +     /*
> > +      * 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)) {
> >               trace_binder_unmap_user_start(alloc, index);
> >
> >               zap_vma_range(vma, page_addr, PAGE_SIZE);
> > @@ -1191,23 +1193,18 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
> >       }
> >
> >       mutex_unlock(&alloc->mutex);
> > -     if (mm_locked)
> > -             mmap_read_unlock(mm);
> > -     else
> > +     if (vma)
> >               vma_end_read(vma);
> >       mmput_async(mm);
> >       binder_free_page(page_to_free);
> >
> >       return LRU_REMOVED_RETRY;
> >
> > -err_invalid_vma:
> >       mutex_unlock(&alloc->mutex);
>
> This mutex_unlock() is now dead-code.

Ack. Will remove.

>
> >  err_get_alloc_mutex_failed:
> > -     if (mm_locked)
> > -             mmap_read_unlock(mm);
> > -     else
> > +     if (vma)
> >               vma_end_read(vma);
> > -err_mmap_read_lock_failed:
> > +err_vma_lock_failed:
> >       mmput_async(mm);
> >  err_mmget:
> >       return LRU_SKIP;
> > --
> > 2.55.0.654.g21b8a5bc05-goog
> >