bcachefs: BTREE_ITER_NOPRESERVE

This adds a flag to not mark the initial btree_path as preserve, for
paths that we expect to be cheap to reconstitute if necessary - this
solves a btree_path overflow caused by need_whiteout_for_snapshot().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_iter.c

@@ -1807,12 +1807,14 @@ static struct btree_path *btree_path_alloc(struct btree_trans *trans,
 	return path;
 }
 
-struct btree_path *bch2_path_get(struct btree_trans *trans, bool cached,
+struct btree_path *bch2_path_get(struct btree_trans *trans,
 				 enum btree_id btree_id, struct bpos pos,
 				 unsigned locks_want, unsigned level,
-				 bool intent)
+				 unsigned flags)
 {
 	struct btree_path *path, *path_pos = NULL;
+	bool cached = flags & BTREE_ITER_CACHED;
+	bool intent = flags & BTREE_ITER_INTENT;
 	int i;
 
 	BUG_ON(trans->restarted);
@@ -1836,7 +1838,6 @@ struct btree_path *bch2_path_get(struct btree_trans *trans, bool cached,
 	    path_pos->level	== level) {
 		__btree_path_get(path_pos, intent);
 		path = btree_path_set_pos(trans, path_pos, pos, intent);
-		path->preserve = true;
 	} else {
 		path = btree_path_alloc(trans, path_pos);
 		path_pos = NULL;
@@ -1845,7 +1846,6 @@ struct btree_path *bch2_path_get(struct btree_trans *trans, bool cached,
 		path->pos			= pos;
 		path->btree_id			= btree_id;
 		path->cached			= cached;
-		path->preserve			= true;
 		path->uptodate			= BTREE_ITER_NEED_TRAVERSE;
 		path->should_be_locked		= false;
 		path->level			= level;
@@ -1860,6 +1860,9 @@ struct btree_path *bch2_path_get(struct btree_trans *trans, bool cached,
 		trans->paths_sorted		= false;
 	}
 
+	if (!(flags & BTREE_ITER_NOPRESERVE))
+		path->preserve = true;
+
 	if (path->intent_ref)
 		locks_want = max(locks_want, level + 1);
 
@@ -2642,13 +2645,8 @@ static void __bch2_trans_iter_init(struct btree_trans *trans,
 	iter->k.p	= pos;
 	iter->k.size	= 0;
 
-	iter->path = bch2_path_get(trans,
-				   flags & BTREE_ITER_CACHED,
-				   btree_id,
-				   iter->pos,
-				   locks_want,
-				   depth,
-				   flags & BTREE_ITER_INTENT);
+	iter->path = bch2_path_get(trans, btree_id, iter->pos,
+				   locks_want, depth, flags);
 }
 
 void bch2_trans_iter_init(struct btree_trans *trans,

fs/bcachefs/btree_iter.h

@@ -159,8 +159,8 @@ bch2_btree_path_make_mut(struct btree_trans *trans,
 
 int __must_check bch2_btree_path_traverse(struct btree_trans *,
 					  struct btree_path *, unsigned);
-struct btree_path *bch2_path_get(struct btree_trans *, bool, enum btree_id,
-				 struct bpos, unsigned, unsigned, bool);
+struct btree_path *bch2_path_get(struct btree_trans *, enum btree_id, struct bpos,
+				 unsigned, unsigned, unsigned);
 inline struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *, struct bkey *);
 
 #ifdef CONFIG_BCACHEFS_DEBUG

fs/bcachefs/btree_types.h

@@ -210,6 +210,7 @@ struct btree_node_iter {
 #define __BTREE_ITER_ALL_SNAPSHOTS	(1 << 11)
 #define BTREE_ITER_ALL_SNAPSHOTS	(1 << 12)
 #define BTREE_ITER_FILTER_SNAPSHOTS	(1 << 13)
+#define BTREE_ITER_NOPRESERVE		(1 << 14)
 
 enum btree_path_uptodate {
 	BTREE_ITER_UPTODATE		= 0,

fs/bcachefs/btree_update_interior.c

@@ -1609,8 +1609,8 @@ int __bch2_foreground_maybe_merge(struct btree_trans *trans,
 		? bpos_predecessor(b->data->min_key)
 		: bpos_successor(b->data->max_key);
 
-	sib_path = bch2_path_get(trans, false, path->btree_id,
-				 sib_pos, U8_MAX, level, true);
+	sib_path = bch2_path_get(trans, path->btree_id, sib_pos,
+				 U8_MAX, level, BTREE_ITER_INTENT);
 	ret = bch2_btree_path_traverse(trans, sib_path, false);
 	if (ret)
 		goto err;

fs/bcachefs/btree_update_leaf.c

@@ -1297,7 +1297,8 @@ static int need_whiteout_for_snapshot(struct btree_trans *trans,
 	pos.snapshot++;
 
 	for_each_btree_key_norestart(trans, iter, btree_id, pos,
-			   BTREE_ITER_ALL_SNAPSHOTS, k, ret) {
+			   BTREE_ITER_ALL_SNAPSHOTS|
+			   BTREE_ITER_NOPRESERVE, k, ret) {
 		if (bkey_cmp(k.k->p, pos))
 			break;