Contention on the xarray lock when multiple threads are adding to the
same xarray can be mitigated by providing a way to add entries in
bulk.
Allow a caller to allocate and populate an xarray node outside of
the target xarray and then only take the xarray lock long enough to
import the node into it.
Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
---
Documentation/core-api/xarray.rst | 8 +++
include/linux/xarray.h | 2 +
lib/test_xarray.c | 45 +++++++++++++++++
lib/xarray.c | 100 ++++++++++++++++++++++++++++++++++++++
4 files changed, 155 insertions(+)
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
index 640934b6f7b4..659e10df8901 100644
--- a/Documentation/core-api/xarray.rst
+++ b/Documentation/core-api/xarray.rst
@@ -444,6 +444,14 @@ called each time the XArray updates a node. This is used by the page
cache workingset code to maintain its list of nodes which contain only
shadow entries.
+xas_export_node() is used to remove and return a node from an XArray
+while xas_import_node() is used to add a node to an XArray. Together
+these can be used, for example, to reduce lock contention when multiple
+threads are updating an XArray by allowing a caller to allocate and
+populate a node outside of the target XArray in a local XArray, export
+the node, and then take the target XArray lock just long enough to import
+the node.
+
Multi-Index Entries
-------------------
diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 14c893433139..73bd8ccc4424 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1504,6 +1504,8 @@ bool xas_nomem(struct xa_state *, gfp_t);
void xas_pause(struct xa_state *);
void xas_create_range(struct xa_state *);
+struct xa_node *xas_export_node(struct xa_state *xas);
+void xas_import_node(struct xa_state *xas, struct xa_node *node);
/**
* xas_reload() - Refetch an entry from the xarray.
diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index d4f97925dbd8..5cfaa1720cc1 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1682,6 +1682,50 @@ static noinline void check_destroy(struct xarray *xa)
#endif
}
+static noinline void check_export_import_1(struct xarray *xa,
+ unsigned long index, unsigned int order)
+{
+ int xa_shift = order + XA_CHUNK_SHIFT - (order % XA_CHUNK_SHIFT);
+ XA_STATE(xas, xa, index);
+ struct xa_node *node;
+ unsigned long i;
+
+ xa_store_many_order(xa, index, xa_shift);
+
+ xas_lock(&xas);
+ xas_set_order(&xas, index, xa_shift);
+ node = xas_export_node(&xas);
+ xas_unlock(&xas);
+
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ do {
+ xas_lock(&xas);
+ xas_set_order(&xas, index, xa_shift);
+ xas_import_node(&xas, node);
+ xas_unlock(&xas);
+ } while (xas_nomem(&xas, GFP_KERNEL));
+
+ for (i = index; i < index + (1UL << xa_shift); i++)
+ xa_erase_index(xa, i);
+
+ XA_BUG_ON(xa, !xa_empty(xa));
+}
+
+static noinline void check_export_import(struct xarray *xa)
+{
+ unsigned int order;
+ unsigned int max_order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 12 : 1;
+
+ for (order = 0; order < max_order; order += XA_CHUNK_SHIFT) {
+ int xa_shift = order + XA_CHUNK_SHIFT;
+ unsigned long j;
+
+ for (j = 0; j < XA_CHUNK_SIZE; j++)
+ check_export_import_1(xa, j << xa_shift, order);
+ }
+}
+
static DEFINE_XARRAY(array);
static int xarray_checks(void)
@@ -1712,6 +1756,7 @@ static int xarray_checks(void)
check_workingset(&array, 0);
check_workingset(&array, 64);
check_workingset(&array, 4096);
+ check_export_import(&array);
printk("XArray: %u of %u tests passed\n", tests_passed, tests_run);
return (tests_run == tests_passed) ? 0 : -EINVAL;
diff --git a/lib/xarray.c b/lib/xarray.c
index e9e641d3c0c3..478925780e87 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -507,6 +507,30 @@ static void xas_delete_node(struct xa_state *xas)
xas_shrink(xas);
}
+static void xas_unlink_node(struct xa_state *xas)
+{
+ struct xa_node *node = xas->xa_node;
+ struct xa_node *parent;
+
+ parent = xa_parent_locked(xas->xa, node);
+ xas->xa_node = parent;
+ xas->xa_offset = node->offset;
+
+ if (!parent) {
+ xas->xa->xa_head = NULL;
+ xas->xa_node = XAS_BOUNDS;
+ return;
+ }
+
+ parent->slots[xas->xa_offset] = NULL;
+ parent->count--;
+ XA_NODE_BUG_ON(parent, parent->count > XA_CHUNK_SIZE);
+
+ xas_update(xas, parent);
+
+ xas_delete_node(xas);
+}
+
/**
* xas_free_nodes() - Free this node and all nodes that it references
* @xas: Array operation state.
@@ -1540,6 +1564,82 @@ static void xas_set_range(struct xa_state *xas, unsigned long first,
}
/**
+ * xas_export_node() - remove and return a node from an XArray
+ * @xas: XArray operation state
+ *
+ * The range covered by @xas must be aligned to and cover a single node
+ * at any level of the tree.
+ *
+ * Return: On success, returns the removed node. If the range is invalid,
+ * returns %NULL and sets -EINVAL in @xas. Otherwise returns %NULL if the
+ * node does not exist.
+ */
+struct xa_node *xas_export_node(struct xa_state *xas)
+{
+ struct xa_node *node;
+
+ if (!xas->xa_shift || xas->xa_sibs) {
+ xas_set_err(xas, -EINVAL);
+ return NULL;
+ }
+
+ xas->xa_shift -= XA_CHUNK_SHIFT;
+
+ if (!xas_find(xas, xas->xa_index))
+ return NULL;
+ node = xas->xa_node;
+ xas_unlink_node(xas);
+ node->parent = NULL;
+
+ return node;
+}
+
+/**
+ * xas_import_node() - add a node to an XArray
+ * @xas: XArray operation state
+ * @node: The node to add
+ *
+ * The range covered by @xas must be aligned to and cover a single node
+ * at any level of the tree. No nodes should already exist within the
+ * range.
+ * Sets an error in @xas if the range is invalid or xas_create() fails
+ */
+void xas_import_node(struct xa_state *xas, struct xa_node *node)
+{
+ struct xa_node *parent = NULL;
+ void __rcu **slot = &xas->xa->xa_head;
+ int count = 0;
+
+ if (!xas->xa_shift || xas->xa_sibs) {
+ xas_set_err(xas, -EINVAL);
+ return;
+ }
+
+ if (xas->xa_index || xa_head_locked(xas->xa)) {
+ xas_set_order(xas, xas->xa_index, node->shift + XA_CHUNK_SHIFT);
+ xas_create(xas, true);
+
+ if (xas_invalid(xas))
+ return;
+
+ parent = xas->xa_node;
+ }
+
+ if (parent) {
+ slot = &parent->slots[xas->xa_offset];
+ node->offset = xas->xa_offset;
+ count++;
+ }
+
+ RCU_INIT_POINTER(node->parent, parent);
+ node->array = xas->xa;
+
+ rcu_assign_pointer(*slot, xa_mk_node(node));
+
+ update_node(xas, parent, count, 0);
+}
+
+/**
* xa_store_range() - Store this entry at a range of indices in the XArray.
* @xa: XArray.
* @first: First index to affect.
--
2.13.3
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