bcachefs: Use a heap for handling overwrites in btree node scan

Fix an O(n^2) issue when we find many overlapping (overwritten) btree
nodes - especially when one node overwrites many smaller nodes.

This was discovered to be an issue with the bcachefs
merge_torture_flakey test - if we had a large btree that was then
emptied, the number of difficult overwrites can be unbounded.

Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_node_scan.c

@@ -12,6 +12,7 @@
 #include "recovery_passes.h"
 
 #include <linux/kthread.h>
+#include <linux/min_heap.h>
 #include <linux/sort.h>
 
 struct find_btree_nodes_worker {
@@ -31,8 +32,6 @@ static void found_btree_node_to_text(struct printbuf *out, struct bch_fs *c, con
 
 	if (n->range_updated)
 		prt_str(out, " range updated");
-	if (n->overwritten)
-		prt_str(out, " overwritten");
 
 	for (unsigned i = 0; i < n->nr_ptrs; i++) {
 		prt_char(out, ' ');
@@ -140,6 +139,24 @@ static int found_btree_node_cmp_pos(const void *_l, const void *_r)
 	       -found_btree_node_cmp_time(l, r);
 }
 
+static inline bool found_btree_node_cmp_pos_less(const void *l, const void *r, void *arg)
+{
+	return found_btree_node_cmp_pos(l, r) < 0;
+}
+
+static inline void found_btree_node_swap(void *_l, void *_r, void *arg)
+{
+	struct found_btree_node *l = _l;
+	struct found_btree_node *r = _r;
+
+	swap(*l, *r);
+}
+
+static const struct min_heap_callbacks found_btree_node_heap_cbs = {
+	.less = found_btree_node_cmp_pos_less,
+	.swp = found_btree_node_swap,
+};
+
 static void try_read_btree_node(struct find_btree_nodes *f, struct bch_dev *ca,
 				struct bio *bio, struct btree_node *bn, u64 offset)
 {
@@ -295,55 +312,48 @@ static int read_btree_nodes(struct find_btree_nodes *f)
 	return f->ret ?: ret;
 }
 
-static void bubble_up(struct found_btree_node *n, struct found_btree_node *end)
+static bool nodes_overlap(const struct found_btree_node *l,
+			  const struct found_btree_node *r)
 {
-	while (n + 1 < end &&
-	       found_btree_node_cmp_pos(n, n + 1) > 0) {
-		swap(n[0], n[1]);
-		n++;
-	}
+	return (l->btree_id	== r->btree_id &&
+		l->level	== r->level &&
+		bpos_gt(l->max_key, r->min_key));
 }
 
 static int handle_overwrites(struct bch_fs *c,
-			     struct found_btree_node *start,
-			     struct found_btree_node *end)
+			     struct found_btree_node *l,
+			     found_btree_nodes *nodes_heap)
 {
-	struct found_btree_node *n;
-again:
-	for (n = start + 1;
-	     n < end &&
-	     n->btree_id	== start->btree_id &&
-	     n->level		== start->level &&
-	     bpos_lt(n->min_key, start->max_key);
-	     n++)  {
-		int cmp = found_btree_node_cmp_time(start, n);
+	struct found_btree_node *r;
+
+	while ((r = min_heap_peek(nodes_heap)) &&
+	       nodes_overlap(l, r)) {
+		int cmp = found_btree_node_cmp_time(l, r);
 
 		if (cmp > 0) {
-			if (bpos_cmp(start->max_key, n->max_key) >= 0)
-				n->overwritten = true;
+			if (bpos_cmp(l->max_key, r->max_key) >= 0)
+				min_heap_pop(nodes_heap, &found_btree_node_heap_cbs, NULL);
 			else {
-				n->range_updated = true;
-				n->min_key = bpos_successor(start->max_key);
-				n->range_updated = true;
-				bubble_up(n, end);
-				goto again;
+				r->range_updated = true;
+				r->min_key = bpos_successor(l->max_key);
+				r->range_updated = true;
+				min_heap_sift_down(nodes_heap, 0, &found_btree_node_heap_cbs, NULL);
 			}
 		} else if (cmp < 0) {
-			BUG_ON(bpos_cmp(n->min_key, start->min_key) <= 0);
+			BUG_ON(bpos_eq(l->min_key, r->min_key));
 
-			start->max_key = bpos_predecessor(n->min_key);
-			start->range_updated = true;
-		} else if (n->level) {
-			n->overwritten = true;
+			l->max_key = bpos_predecessor(r->min_key);
+			l->range_updated = true;
+		} else if (r->level) {
+			min_heap_pop(nodes_heap, &found_btree_node_heap_cbs, NULL);
 		} else {
-			if (bpos_cmp(start->max_key, n->max_key) >= 0)
-				n->overwritten = true;
+			if (bpos_cmp(l->max_key, r->max_key) >= 0)
+				min_heap_pop(nodes_heap, &found_btree_node_heap_cbs, NULL);
 			else {
-				n->range_updated = true;
-				n->min_key = bpos_successor(start->max_key);
-				n->range_updated = true;
-				bubble_up(n, end);
-				goto again;
+				r->range_updated = true;
+				r->min_key = bpos_successor(l->max_key);
+				r->range_updated = true;
+				min_heap_sift_down(nodes_heap, 0, &found_btree_node_heap_cbs, NULL);
 			}
 		}
 	}
@@ -355,6 +365,7 @@ int bch2_scan_for_btree_nodes(struct bch_fs *c)
 {
 	struct find_btree_nodes *f = &c->found_btree_nodes;
 	struct printbuf buf = PRINTBUF;
+	found_btree_nodes nodes_heap = {};
 	size_t dst;
 	int ret = 0;
 
@@ -409,29 +420,57 @@ int bch2_scan_for_btree_nodes(struct bch_fs *c)
 		bch2_print_string_as_lines(KERN_INFO, buf.buf);
 	}
 
-	dst = 0;
-	darray_for_each(f->nodes, i) {
-		if (i->overwritten)
-			continue;
+	swap(nodes_heap, f->nodes);
 
-		ret = handle_overwrites(c, i, &darray_top(f->nodes));
+	{
+		/* darray must have same layout as a heap */
+		min_heap_char real_heap;
+		BUILD_BUG_ON(sizeof(nodes_heap.nr)	!= sizeof(real_heap.nr));
+		BUILD_BUG_ON(sizeof(nodes_heap.size)	!= sizeof(real_heap.size));
+		BUILD_BUG_ON(offsetof(found_btree_nodes, nr)	!= offsetof(min_heap_char, nr));
+		BUILD_BUG_ON(offsetof(found_btree_nodes, size)	!= offsetof(min_heap_char, size));
+	}
+
+	min_heapify_all(&nodes_heap, &found_btree_node_heap_cbs, NULL);
+
+	if (nodes_heap.nr) {
+		ret = darray_push(&f->nodes, *min_heap_peek(&nodes_heap));
 		if (ret)
 			goto err;
 
-		BUG_ON(i->overwritten);
-		f->nodes.data[dst++] = *i;
+		min_heap_pop(&nodes_heap, &found_btree_node_heap_cbs, NULL);
 	}
-	f->nodes.nr = dst;
 
-	if (c->opts.verbose) {
+	while (true) {
+		ret = handle_overwrites(c, &darray_last(f->nodes), &nodes_heap);
+		if (ret)
+			goto err;
+
+		if (!nodes_heap.nr)
+			break;
+
+		ret = darray_push(&f->nodes, *min_heap_peek(&nodes_heap));
+		if (ret)
+			goto err;
+
+		min_heap_pop(&nodes_heap, &found_btree_node_heap_cbs, NULL);
+	}
+
+	for (struct found_btree_node *n = f->nodes.data; n < &darray_last(f->nodes); n++)
+		BUG_ON(nodes_overlap(n, n + 1));
+
+	if (0 && c->opts.verbose) {
 		printbuf_reset(&buf);
 		prt_printf(&buf, "%s: nodes found after overwrites:\n", __func__);
 		found_btree_nodes_to_text(&buf, c, f->nodes);
 		bch2_print_string_as_lines(KERN_INFO, buf.buf);
+	} else {
+		bch_info(c, "btree node scan found %zu nodes after overwrites", f->nodes.nr);
 	}
 
 	eytzinger0_sort(f->nodes.data, f->nodes.nr, sizeof(f->nodes.data[0]), found_btree_node_cmp_pos, NULL);
 err:
+	darray_exit(&nodes_heap);
 	printbuf_exit(&buf);
 	return ret;
 }

fs/bcachefs/btree_node_scan_types.h

@@ -6,7 +6,6 @@
 
 struct found_btree_node {
 	bool			range_updated:1;
-	bool			overwritten:1;
 	u8			btree_id;
 	u8			level;
 	unsigned		sectors_written;