[PATCH] binder_alloc: Move alloc_page() out of mmap_rwsem to reduce the lock duration

[Barry Song]

From: Barry Song <v-songbaohua@xxxxxxxx>

The mmap_write_lock() can block all access to the VMAs, for example page
faults. Performing memory allocation while holding this lock may trigger
direct reclamation, leading to others being queued in the rwsem for an
extended period.
We've observed that the allocation can sometimes take more than 300ms,
significantly blocking other threads. The user interface sometimes
becomes less responsive as a result. To prevent this, let's move the
allocation outside of the write lock.
A potential side effect could be an extra alloc_page() for the second
thread executing binder_install_single_page() while the first thread
has done it earlier. However, according to Tangquan's 48-hour profiling
using monkey, the likelihood of this occurring is minimal, with a ratio
of only 1 in 2400. Compared to the significantly costly rwsem, this is
negligible.
On the other hand, holding a write lock without making any VMA
modifications appears questionable and likely incorrect. While this
patch focuses on reducing the lock duration, future updates may aim
to eliminate the write lock entirely.

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: "Arve Hjønnevåg" <arve@xxxxxxxxxxx>
Cc: Todd Kjos <tkjos@xxxxxxxxxxx>
Cc: Martijn Coenen <maco@xxxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Carlos Llamas <cmllamas@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Tested-by: Tangquan Zheng <zhengtangquan@xxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 drivers/android/binder_alloc.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index b3acbc4174fb..f20074e23a7c 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -227,13 +227,23 @@ static int binder_install_single_page(struct binder_alloc *alloc,
 	if (!mmget_not_zero(alloc->mm))
 		return -ESRCH;
 
+	/*
+	 * Don't allocate page in mmap_write_lock, this can block
+	 * mmap_rwsem for a long time; Meanwhile, allocation failure
+	 * doesn't necessarily need to return -ENOMEM, if lru_page
+	 * has been installed, we can still return 0(success).
+	 */
+	page = alloc_page(GFP_KERNEL | __GFP_HIGHMEM | __GFP_ZERO);
+
 	/*
 	 * Protected with mmap_sem in write mode as multiple tasks
 	 * might race to install the same page.
 	 */
 	mmap_write_lock(alloc->mm);
-	if (binder_get_installed_page(lru_page))
+	if (binder_get_installed_page(lru_page)) {
+		ret = 1;
 		goto out;
+	}
 
 	if (!alloc->vma) {
 		pr_err("%d: %s failed, no vma\n", alloc->pid, __func__);
@@ -241,7 +251,6 @@ static int binder_install_single_page(struct binder_alloc *alloc,
 		goto out;
 	}
 
-	page = alloc_page(GFP_KERNEL | __GFP_HIGHMEM | __GFP_ZERO);
 	if (!page) {
 		pr_err("%d: failed to allocate page\n", alloc->pid);
 		ret = -ENOMEM;
@@ -252,7 +261,6 @@ static int binder_install_single_page(struct binder_alloc *alloc,
 	if (ret) {
 		pr_err("%d: %s failed to insert page at offset %lx with %d\n",
 		       alloc->pid, __func__, addr - alloc->buffer, ret);
-		__free_page(page);
 		ret = -ENOMEM;
 		goto out;
 	}
@@ -262,7 +270,9 @@ static int binder_install_single_page(struct binder_alloc *alloc,
 out:
 	mmap_write_unlock(alloc->mm);
 	mmput_async(alloc->mm);
-	return ret;
+	if (ret && page)
+		__free_page(page);
+	return ret < 0 ? ret : 0;
 }
 
 static int binder_install_buffer_pages(struct binder_alloc *alloc,
-- 
2.39.3 (Apple Git-146)