[PATCH v6 2/5] binder: Make shrinker rely solely on per-VMA lock

From: Dave Hansen <dave.hansen@linux.intel.com>

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.

While at it we also make the handling of the cases where the original
binder VMA is gone consistent. There are two cases to consider when
Binder VMA is gone:
1. there is no VMA at that location anymore.
2. there is now another unrelated VMA at that location.

Before this change we handle case 1 by having the shrinker proceed to
free the page, and just skip the zap_vma_range() call. And we handle
case 2 by having the shrinker return LRU_SKIP. While either behavior
is acceptable, we need to handle them in a consistent way. Handle both
cases by freeing the page without touching the VMA (skipping the
zap_vma_range()).

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@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Liam R. Howlett <redacted>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: linux-mm@kvack.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: netdev@vger.kernel.org
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 drivers/android/binder_alloc.c | 46 ++++++++++++++++------------------
 1 file changed, 21 insertions(+), 25 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index e4488ad86a65..fcb744088e77 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'.
+	 */
 	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;
 	}
 
 	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;
-
 	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 +1193,17 @@ 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);
 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;

-- 
2.55.0.691.gc56d675ccc-goog