[PATCH v6 44/99] shmem: Convert shmem_tag_pins to XArray

[Matthew Wilcox]

From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

Simplify the locking by taking the spinlock while we walk the tree on
the assumption that many acquires and releases of the lock will be
worse than holding the lock for a (potentially) long time.

We could replicate the same locking behaviour with the xarray, but would
have to be careful that the xa_node wasn't RCU-freed under us before we
took the lock.

Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
---
 mm/shmem.c | 39 ++++++++++++++++-----------------------
 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index ce285ae635ea..2f41c7ceea18 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2601,35 +2601,28 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
 
 static void shmem_tag_pins(struct address_space *mapping)
 {
-	struct radix_tree_iter iter;
-	void **slot;
-	pgoff_t start;
+	XA_STATE(xas, &mapping->pages, 0);
 	struct page *page;
+	unsigned int tagged = 0;
 
 	lru_add_drain();
-	start = 0;
-	rcu_read_lock();
 
-	radix_tree_for_each_slot(slot, &mapping->pages, &iter, start) {
-		page = radix_tree_deref_slot(slot);
-		if (!page || radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page)) {
-				slot = radix_tree_iter_retry(&iter);
-				continue;
-			}
-		} else if (page_count(page) - page_mapcount(page) > 1) {
-			xa_lock_irq(&mapping->pages);
-			radix_tree_tag_set(&mapping->pages, iter.index,
-					   SHMEM_TAG_PINNED);
-			xa_unlock_irq(&mapping->pages);
-		}
+	xas_lock_irq(&xas);
+	xas_for_each(&xas, page, ULONG_MAX) {
+		if (xa_is_value(page))
+			continue;
+		if (page_count(page) - page_mapcount(page) > 1)
+			xas_set_tag(&xas, SHMEM_TAG_PINNED);
 
-		if (need_resched()) {
-			slot = radix_tree_iter_resume(slot, &iter);
-			cond_resched_rcu();
-		}
+		if (++tagged % XA_CHECK_SCHED)
+			continue;
+
+		xas_pause(&xas);
+		xas_unlock_irq(&xas);
+		cond_resched();
+		xas_lock_irq(&xas);
 	}
-	rcu_read_unlock();
+	xas_unlock_irq(&xas);
 }
 
 /*
-- 
2.15.1