maple_tree: add sufficient height

In order to support rebalancing and spanning stores using less than the
worst case number of nodes, we need to track more than just the vacant
height.  Using only vacant height to reduce the worst case maple node
allocation count can lead to a shortcoming of nodes in the following
scenarios.

For rebalancing writes, when a leaf node becomes insufficient, it may be
combined with a sibling into a single node.  This means that the parent
node which has entries for this children will lose one entry.  If this
parent node was just meeting the minimum entries, losing one entry will
now cause this parent node to be insufficient.  This leads to a cascading
operation of rebalancing at different levels and can lead to more node
allocations than simply using vacant height can return.

For spanning writes, a similar situation occurs.  At the location at which
a spanning write is detected, the number of ancestor nodes may similarly
need to rebalanced into a smaller number of nodes and the same cascading
situation could occur.

To use less than the full height of the tree for the number of
allocations, we also need to track the height at which a non-leaf node
cannot become insufficient.  This means even if a rebalance occurs to a
child of this node, it currently has enough entries that it can lose one
without any further action.  This field is stored in the maple write state
as sufficient height.  In mas_prealloc_calc() when figuring out how many
nodes to allocate, we check if the the vacant node is lower in the tree
than a sufficient node (has a larger value).  If it is, we cannot use the
vacant height and must use the difference in the height and sufficient
height as the basis for the number of nodes needed.

Link: https://lkml.kernel.org/r/20250227204823.758784-6-sidhartha.kumar@oracle.com
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
index 7d777aa2d9ed968f16075dbbe2c21493004bc037..37dc9525dff67b7c11e161cd8071871273caaa85 100644 (file)
--- a/include/linux/maple_tree.h
+++ b/include/linux/maple_tree.h
@@ -464,6 +464,7 @@ struct ma_wr_state {
        void *entry;                    /* The entry to write */
        void *content;                  /* The existing entry that is being overwritten */
        unsigned char vacant_height;    /* Depth of lowest node with free space */
+       unsigned char sufficient_height;/* Depth of lowest node with min sufficiency + 1 nodes */
 };
 
 #define mas_lock(mas)           spin_lock(&((mas)->tree->ma_lock))
@@ -499,7 +500,8 @@ struct ma_wr_state {
                .mas = ma_state,                                        \
                .content = NULL,                                        \
                .entry = wr_entry,                                      \
-               .vacant_height = 0                                      \
+               .vacant_height = 0,                                     \
+               .sufficient_height = 0                                  \
        }
 
 #define MA_TOPIARY(name, tree)                                         \

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 8cea953a0cb5672cfee16eebb99f524465734875..0d1b55b1dc1293c00c919bfac0c950d27612fbd7 100644 (file)
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3556,6 +3556,13 @@ static bool mas_wr_walk(struct ma_wr_state *wr_mas)
                if (mas->end < mt_slots[wr_mas->type] - 1)
                        wr_mas->vacant_height = mas->depth + 1;
 
+               if (ma_is_root(mas_mn(mas))) {
+                       /* root needs more than 2 entries to be sufficient + 1 */
+                       if (mas->end > 2)
+                               wr_mas->sufficient_height = 1;
+               } else if (mas->end > mt_min_slots[wr_mas->type] + 1)
+                       wr_mas->sufficient_height = mas->depth + 1;
+
                mas_wr_walk_traverse(wr_mas);
        }
 
@@ -4191,13 +4198,19 @@ static inline int mas_prealloc_calc(struct ma_wr_state *wr_mas, void *entry)
                        ret = 0;
                break;
        case wr_spanning_store:
-               WARN_ON_ONCE(ret != height * 3 + 1);
+               if (wr_mas->sufficient_height < wr_mas->vacant_height)
+                       ret = (height - wr_mas->sufficient_height) * 3 + 1;
+               else
+                       ret = delta * 3 + 1;
                break;
        case wr_split_store:
                ret = delta * 2 + 1;
                break;
        case wr_rebalance:
-               ret = height * 2 + 1;
+               if (wr_mas->sufficient_height < wr_mas->vacant_height)
+                       ret = (height - wr_mas->sufficient_height) * 2 + 1;
+               else
+                       ret = delta * 2 + 1;
                break;
        case wr_node_store:
                ret = mt_in_rcu(mas->tree) ? 1 : 0;

diff --git a/tools/testing/radix-tree/maple.c b/tools/testing/radix-tree/maple.c
index 5950a7c9b27f6734a95e676d788bb198ce4194de..dc8e40b94c3f757fad4509293526e81bf6758d08 100644 (file)
--- a/tools/testing/radix-tree/maple.c
+++ b/tools/testing/radix-tree/maple.c
@@ -36311,6 +36311,30 @@ static noinline void __init check_mtree_dup(struct maple_tree *mt)
 
 extern void test_kmem_cache_bulk(void);
 
+/*
+ * Test to check the path of a spanning rebalance which results in
+ * a collapse where the rebalancing of the child node leads to
+ * insufficieny in the parent node.
+ */
+static void check_collapsing_rebalance(struct maple_tree *mt)
+{
+       int i = 0;
+       MA_STATE(mas, mt, ULONG_MAX, ULONG_MAX);
+
+       /* create a height 4 tree */
+       while (mt_height(mt) < 4) {
+               mtree_store_range(mt, i, i + 10, xa_mk_value(i), GFP_KERNEL);
+               i += 9;
+       }
+
+       /* delete all entries one at a time, starting from the right */
+       do {
+               mas_erase(&mas);
+       } while (mas_prev(&mas, 0) != NULL);
+
+       mtree_unlock(mt);
+}
+
 /* callback function used for check_nomem_writer_race() */
 static void writer2(void *maple_tree)
 {
@@ -36477,6 +36501,10 @@ void farmer_tests(void)
        check_spanning_write(&tree);
        mtree_destroy(&tree);
 
+       mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
+       check_collapsing_rebalance(&tree);
+       mtree_destroy(&tree);
+
        mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
        check_null_expand(&tree);
        mtree_destroy(&tree);