[PATCH 1/2] mm/filemap: use xas_try_split() in __filemap_add_folio().

[Zi Yan <ziy@nvidia.com>]

During __filemap_add_folio(), a shadow entry is covering n slots and a
folio covers m slots with m < n is to be added. Instead of splitting all
n slots, only the m slots covered by the folio need to be split and the
remaining n-m shadow entries can be retained with orders ranging from m to
n-1. This method only requires (n/XA_CHUNK_SHIFT) - (m/XA_CHUNK_SHIFT)
new xa_nodes instead of
(n % XA_CHUNK_SHIFT) * ((n/XA_CHUNK_SHIFT) - (m/XA_CHUNK_SHIFT)) new
xa_nodes, compared to the original xas_split_alloc() + xas_split() one.
For example, to insert an order-0 folio when an order-9 shadow entry is
present (assuming XA_CHUNK_SHIFT is 6), 1 xa_node is needed instead of 8.

xas_try_split_min_order() is introduced to reduce the number of calls to
xas_try_split() during split.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/xarray.h |  7 +++++++
 lib/xarray.c           | 25 +++++++++++++++++++++++
 mm/filemap.c           | 46 +++++++++++++++++-------------------------
 3 files changed, 51 insertions(+), 27 deletions(-)

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 9eb8c7425090..6ef3d682b189 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1557,6 +1557,7 @@ void xas_split(struct xa_state *, void *entry, unsigned int order);
 void xas_split_alloc(struct xa_state *, void *entry, unsigned int order, gfp_t);
 void xas_try_split(struct xa_state *xas, void *entry, unsigned int order,
 		gfp_t gfp);
+unsigned int xas_try_split_min_order(unsigned int order);
 #else
 static inline int xa_get_order(struct xarray *xa, unsigned long index)
 {
@@ -1583,6 +1584,12 @@ static inline void xas_try_split(struct xa_state *xas, void *entry,
 		unsigned int order, gfp_t gfp)
 {
 }
+
+static inline unsigned int xas_try_split_min_order(unsigned int order)
+{
+	return 0;
+}
+
 #endif
 
 /**
diff --git a/lib/xarray.c b/lib/xarray.c
index c38beca77830..1805fde1c361 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1133,6 +1133,28 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 }
 EXPORT_SYMBOL_GPL(xas_split);
 
+/**
+ * xas_try_split_min_order() - Minimal split order xas_try_split() can accept
+ * @order: Current entry order.
+ *
+ * xas_try_split() can split a multi-index entry to smaller than @order - 1 if
+ * no new xa_node is needed. This function provides the minimal order
+ * xas_try_split() supports.
+ *
+ * Return: the minimal order xas_try_split() supports
+ *
+ * Context: Any context.
+ *
+ */
+unsigned int xas_try_split_min_order(unsigned int order)
+{
+	if (order % XA_CHUNK_SHIFT == 0)
+		return order == 0 ? 0 : order - 1;
+
+	return order - (order % XA_CHUNK_SHIFT);
+}
+EXPORT_SYMBOL_GPL(xas_try_split_min_order);
+
 /**
  * xas_try_split() - Try to split a multi-index entry.
  * @xas: XArray operation state.
@@ -1145,6 +1167,9 @@ EXPORT_SYMBOL_GPL(xas_split);
  * be allocated, the function will use @gfp to get one. If more xa_node are
  * needed, the function gives EINVAL error.
  *
+ * NOTE: use xas_try_split_min_order() to get next split order instead of
+ * @order - 1 if you want to minmize xas_try_split() calls.
+ *
  * Context: Any context.  The caller should hold the xa_lock.
  */
 void xas_try_split(struct xa_state *xas, void *entry, unsigned int order,
diff --git a/mm/filemap.c b/mm/filemap.c
index 804d7365680c..e28a7a623889 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -860,11 +860,10 @@ EXPORT_SYMBOL_GPL(replace_page_cache_folio);
 noinline int __filemap_add_folio(struct address_space *mapping,
 		struct folio *folio, pgoff_t index, gfp_t gfp, void **shadowp)
 {
-	XA_STATE(xas, &mapping->i_pages, index);
-	void *alloced_shadow = NULL;
-	int alloced_order = 0;
+	XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
 	bool huge;
 	long nr;
+	unsigned int forder = folio_order(folio);
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(folio_test_swapbacked(folio), folio);
@@ -873,7 +872,6 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 	mapping_set_update(&xas, mapping);
 
 	VM_BUG_ON_FOLIO(index & (folio_nr_pages(folio) - 1), folio);
-	xas_set_order(&xas, index, folio_order(folio));
 	huge = folio_test_hugetlb(folio);
 	nr = folio_nr_pages(folio);
 
@@ -883,7 +881,7 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 	folio->index = xas.xa_index;
 
 	for (;;) {
-		int order = -1, split_order = 0;
+		int order = -1;
 		void *entry, *old = NULL;
 
 		xas_lock_irq(&xas);
@@ -901,21 +899,26 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 				order = xas_get_order(&xas);
 		}
 
-		/* entry may have changed before we re-acquire the lock */
-		if (alloced_order && (old != alloced_shadow || order != alloced_order)) {
-			xas_destroy(&xas);
-			alloced_order = 0;
-		}
-
 		if (old) {
-			if (order > 0 && order > folio_order(folio)) {
+			if (order > 0 && order > forder) {
+				unsigned int split_order = max(forder,
+						xas_try_split_min_order(order));
+
 				/* How to handle large swap entries? */
 				BUG_ON(shmem_mapping(mapping));
-				if (!alloced_order) {
-					split_order = order;
-					goto unlock;
+
+				while (order > forder) {
+					xas_set_order(&xas, index, split_order);
+					xas_try_split(&xas, old, order,
+						      GFP_NOWAIT);
+					if (xas_error(&xas))
+						goto unlock;
+					order = split_order;
+					split_order =
+						max(xas_try_split_min_order(
+							    split_order),
+						    forder);
 				}
-				xas_split(&xas, old, order);
 				xas_reset(&xas);
 			}
 			if (shadowp)
@@ -939,17 +942,6 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 unlock:
 		xas_unlock_irq(&xas);
 
-		/* split needed, alloc here and retry. */
-		if (split_order) {
-			xas_split_alloc(&xas, old, split_order, gfp);
-			if (xas_error(&xas))
-				goto error;
-			alloced_shadow = old;
-			alloced_order = split_order;
-			xas_reset(&xas);
-			continue;
-		}
-
 		if (!xas_nomem(&xas, gfp))
 			break;
 	}
-- 
2.47.2