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

[Lorenzo Stoakes]

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>

> 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;
>  }
> _