Re: [PATCH 4/6] binder: Remove mmap_lock fallback

[Suren Baghdasaryan]

On Fri, May 8, 2026 at 10:29 AM Lorenzo Stoakes <ljs@xxxxxxxxxx> wrote:
>
> On Wed, Apr 29, 2026 at 11:20:00AM -0700, Dave Hansen wrote:
> >
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >
> > Previously, the per-VMA locking could fail in the face of writers
> > which necessitate a fallback to mmap_lock. The new
> > lock_vma_under_rcu_wait() will wait for writers instead of failing.
> >
> > Use the new helper. Wait for writers. Remove the fallback to mmap_lock.
> >
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> LGTM in principal, though again not a binder dev so just an A-b :)
>
> Acked-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

CC'ing Carlos to keep him informed on binder changes.

>
> > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> > Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > ---
> >
> >  b/drivers/android/binder_alloc.c |   17 +++++------------
> >  1 file changed, 5 insertions(+), 12 deletions(-)
> >
> > diff -puN drivers/android/binder_alloc.c~binder-vma-waiter drivers/android/binder_alloc.c
> > --- a/drivers/android/binder_alloc.c~binder-vma-waiter        2026-04-29 11:18:51.307654829 -0700
> > +++ b/drivers/android/binder_alloc.c  2026-04-29 11:18:51.310654944 -0700
> > @@ -259,21 +259,14 @@ static int binder_page_insert(struct bin
> >       struct vm_area_struct *vma;
> >       int ret = -ESRCH;
> >
> > -     /* attempt per-vma lock first */
> > -     vma = lock_vma_under_rcu(mm, addr);
> > -     if (vma) {
> > -             if (binder_alloc_is_mapped(alloc))
> > -                     ret = vm_insert_page(vma, addr, page);
> > -             vma_end_read(vma);
> > +     vma = lock_vma_under_rcu_wait(mm, addr);
>
> Yeah this name is definitely iffy haha!
>
> > +     if (!vma)
> >               return ret;
> > -     }
> >
> > -     /* fall back to mmap_lock */
> > -     mmap_read_lock(mm);
> > -     vma = vma_lookup(mm, addr);
> > -     if (vma && binder_alloc_is_mapped(alloc))
> > +     if (binder_alloc_is_mapped(alloc))
> >               ret = vm_insert_page(vma, addr, page);
> > -     mmap_read_unlock(mm);
> > +
> > +     vma_end_read(vma);
> >
> >       return ret;
> >  }
> > _