Re: [PATCH v2 2/5] binder: Make shrinker rely solely on per-VMA lock

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/11/26 21:59, Suren Baghdasaryan wrote:
> On Thu, Jun 11, 2026 at 12:53 AM Alice Ryhl <aliceryhl@google.com> wrote:
>>
>> >  b/drivers/android/binder_alloc.c |   26 +++++++++-----------------
>> >  1 file changed, 9 insertions(+), 17 deletions(-)
>> >
>> > diff -puN drivers/android/binder_alloc.c~binder-try-vma-lock drivers/android/binder_alloc.c
>> > --- a/drivers/android/binder_alloc.c~binder-try-vma-lock      2026-06-10 15:57:55.274412018 -0700
>> > +++ b/drivers/android/binder_alloc.c  2026-06-10 15:57:55.277412124 -0700
>> > @@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(s
>> >       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))
>> > @@ -1151,15 +1150,12 @@ enum lru_status binder_alloc_free_page(s
>> >       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'.
>> > +      */
>> >       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 (!mutex_trylock(&alloc->mutex))
>> >               goto err_get_alloc_mutex_failed;
>> > @@ -1188,13 +1184,11 @@ enum lru_status binder_alloc_free_page(s
>> >               zap_vma_range(vma, page_addr, PAGE_SIZE);
>> >
>> >               trace_binder_unmap_user_end(alloc, index);
>> > +
>> > +             vma_end_read(vma);
>> >       }
>> >
>> >       mutex_unlock(&alloc->mutex);
>> > -     if (mm_locked)
>> > -             mmap_read_unlock(mm);
>> > -     else
>> > -             vma_end_read(vma);
>> >       mmput_async(mm);
>> >       binder_free_page(page_to_free);
>> >
>> > @@ -1203,11 +1197,9 @@ enum lru_status binder_alloc_free_page(s
>> >  err_invalid_vma:
>> >       mutex_unlock(&alloc->mutex);
>> >  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:

This label is unused btw, which is related to Alice's point.

>> >       mmput_async(mm);
>>
>> If the vma lookup fails because the mmap write lock is held, but the vma
>> actually exists (has not been unmapped), then this code might "successfully"
>> remove the page without invoking zap_vma_range(). This means that the
>> page does not actually get freed and will just hang around forever until
>> the process owning the vma exits or Binder needs this page and maps a
>> new page on top of the page.
> 
> Yeah, I think if lock_vma_under_rcu() returns NULL you just need to
> jump to err_mmap_read_lock_failed, like we currently do if
> mmap_read_trylock() fails.

I don't think that will be enough as well, as the current code AFAICS does
something meaninfgul when mmap_read_trylock() suceeds but vma_lookup returns
NULL because there's no vma at that address. Now we would just assume the
trylock failed even if the reason was that vma lookup found nothing for the
address. The problem is that lock_vma_under_rcu() can't distinguish those
two outcomes, so we would need something that does?