Re: [PATCH v3 2/5] binder: Make shrinker rely solely on per-VMA lock

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 09:04:20AM +0000, Alice Ryhl wrote:
> On Mon, Aug 03, 2026 at 12:10:21PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Sun, Aug 02, 2026 at 02:54:56PM -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.
> > >
> > > 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 | 29 +++++++++++++++--------------
> > >  1 file changed, 15 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > > index e4488ad86a65..84104ba04e30 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))
> > > @@ -1151,14 +1150,20 @@ 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'.
> > > +	 */
> >
> > This makes me wonder whether lock_vma_under_rcu() should really become
> > vma_trylock() at some point in time? :)
> >
> > Or at least have 'trylock' in the name.
> >
> > >  	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;
> >
> > Hmm, it seems a bit odd to me that you also have:
> >
> > 	if (vma && !binder_alloc_is_mapped(alloc))
> > 		goto err_invalid_vma;
> >
> > Below?
> >
> > So you have:
> >
> > Before:
> >
> >                         	|binder_alloc_is_mapped()?
> > 				|yes   	no
> > 			--------|-----------------
> > 	vma is mapped?	yes	|OK	abort
> > 			no	|OK	OK
> >
> > Now:
> >
> >                         	|binder_alloc_is_mapped()?
> > 				|yes   	no
> > 			--------|-----------------
> > 	vma is mapped? maybe	|abort	OK
> > 			yes	|OK	abort
> > 			no	|OK	OK
> >
> > The 'maybe' is because the VMA trylock failed.
> >
> > So the issue is you might have a case where the VMA _is_ mapped but
> > !binder_alloc_is_mapped(), which previously aborted because of the vma &&
> > !binder_alloc_is_mapped() check.
> >
> > It seems like:
> >
> > 	/*
> > 	 * 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;
> >
> > Is testing for a specific scenario 'we found a VMA but it turns out it's
> > invalid' and aborting if so.
> >
> > So either this check should be removed or you should uncondtionally abort if
> > !vma I think?
>
> This check is quite important and can't just be removed. If you remove
> it, there's no guarantee that the vma is one created by Binder. It might
> as well be a VMA from a completely different driver/subsystem, which we
> definitely should not be invoking zap_vma_range() on.
>
> In this case, Binder rules out that scenario by saying that the VMA
> can be mapped exactly once, and once you unmap it or remap it or
> anything like that, Binder sets the 'is_mapped' boolean to false and
> refuses to perform any further VMA operations for this binder fd.
>
> So really this function needs to deal with three scenarios:
>
> 1. The original Binder VMA is still there and we acquired its lock.
> 2. The original Binder VMA is still there, but we could not acquire its
>    lock.
> 3. The original Binder VMA is gone.
>    - Subcase one: there is no VMA at that location anymore.
>    - Subcase two: there is now another unrelated VMA at that location.
>
> In scenario one we can proceed with zapping the page. In scenario two we
> must return LRU_SKIP because we are unable to zap the page. As for
> scenario three, it's a scenario that is possible, but not something that
> needs to work well. It doesn't matter that much whether such pages can
> be reclaimed by the shrinker because userspace shouldn't create this
> scenario to begin with.
>
> But you are right that we currently handle scenario 3 inconsistently. We
> handle subcase one by having the shrinker proceed to free the page, and
> just skip the zap_vma_range() call. And we handle subcase two by having
> the shrinker return LRU_SKIP. Either behavior is acceptable to me, but I
> agree that being consistent would be better.
>
> So what we could do is to remove this check, but then later wrap
> zap_vma_range() in an 'is_mapped' check like this:
>
> 	if (vma && binder_alloc_is_mapped(alloc)) {
> 		zap_vma_range(vma, page_addr, PAGE_SIZE);
> 	}
>
> This way we only LRU_SKIP in case two, and always handle case 3 by
> removing the page from alloc->pages without touching the VMA.

Ah sorry I replied to Suren not noticing you'd responded :)

Thanks for the explanation, much appreciated! Makes sense.

I'm being OCD about it (occupational hazard in kernel development :) but the
inconsitency was concerning there. And agreed it's an edge case.

Suren - that good for the respin?

>
> Alice

--
Cheers, Lorenzo