bcachefs: Don't allocate memory under the btree cache lock

The btree cache lock is needed for reclaiming from the btree node cache,
and memory allocation can potentially spin and sleep (for 100 ms at a
time), so.. don't do that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.c

@@ -72,24 +72,33 @@ static const struct rhashtable_params bch_btree_cache_params = {
 	.obj_cmpfn	= bch2_btree_cache_cmp_fn,
 };
 
-static void btree_node_data_alloc(struct bch_fs *c, struct btree *b, gfp_t gfp)
+static int __btree_node_data_alloc(struct bch_fs *c, struct btree *b, gfp_t gfp)
 {
-	struct btree_cache *bc = &c->btree_cache;
+	BUG_ON(b->data || b->aux_data);
 
 	b->data = kvpmalloc(btree_bytes(c), gfp);
 	if (!b->data)
-		goto err;
+		return -ENOMEM;
 
-	if (bch2_btree_keys_alloc(b, btree_page_order(c), gfp))
-		goto err;
+	if (bch2_btree_keys_alloc(b, btree_page_order(c), gfp)) {
+		kvpfree(b->data, btree_bytes(c));
+		b->data = NULL;
+		return -ENOMEM;
+	}
 
-	bc->used++;
-	list_move(&b->list, &bc->freeable);
-	return;
-err:
-	kvpfree(b->data, btree_bytes(c));
-	b->data = NULL;
-	list_move(&b->list, &bc->freed);
+	return 0;
+}
+
+static void btree_node_data_alloc(struct bch_fs *c, struct btree *b, gfp_t gfp)
+{
+	struct btree_cache *bc = &c->btree_cache;
+
+	if (!__btree_node_data_alloc(c, b, gfp)) {
+		bc->used++;
+		list_move(&b->list, &bc->freeable);
+	} else {
+		list_move(&b->list, &bc->freed);
+	}
 }
 
 static struct btree *btree_node_mem_alloc(struct bch_fs *c, gfp_t gfp)
@@ -525,35 +534,47 @@ struct btree *bch2_btree_node_mem_alloc(struct bch_fs *c)
 	 */
 	list_for_each_entry(b, &bc->freeable, list)
 		if (!btree_node_reclaim(c, b))
-			goto out_unlock;
+			goto got_node;
 
 	/*
 	 * We never free struct btree itself, just the memory that holds the on
 	 * disk node. Check the freed list before allocating a new one:
 	 */
 	list_for_each_entry(b, &bc->freed, list)
-		if (!btree_node_reclaim(c, b)) {
-			btree_node_data_alloc(c, b, __GFP_NOWARN|GFP_NOIO);
-			if (b->data)
-				goto out_unlock;
+		if (!btree_node_reclaim(c, b))
+			goto got_node;
 
-			six_unlock_write(&b->c.lock);
-			six_unlock_intent(&b->c.lock);
+	b = NULL;
+got_node:
+	if (b)
+		list_del_init(&b->list);
+	mutex_unlock(&bc->lock);
+
+	if (!b) {
+		b = kzalloc(sizeof(struct btree), GFP_KERNEL);
+		if (!b)
 			goto err;
-		}
 
-	b = btree_node_mem_alloc(c, __GFP_NOWARN|GFP_NOIO);
-	if (!b)
-		goto err;
+		bkey_btree_ptr_init(&b->key);
+		six_lock_init(&b->c.lock);
+		INIT_LIST_HEAD(&b->list);
+		INIT_LIST_HEAD(&b->write_blocked);
+
+		BUG_ON(!six_trylock_intent(&b->c.lock));
+		BUG_ON(!six_trylock_write(&b->c.lock));
+	}
+
+	if (!b->data) {
+		if (__btree_node_data_alloc(c, b, __GFP_NOWARN|GFP_KERNEL))
+			goto err;
+
+		mutex_lock(&bc->lock);
+		bc->used++;
+		mutex_unlock(&bc->lock);
+	}
 
-	BUG_ON(!six_trylock_intent(&b->c.lock));
-	BUG_ON(!six_trylock_write(&b->c.lock));
-out_unlock:
 	BUG_ON(btree_node_hashed(b));
 	BUG_ON(btree_node_write_in_flight(b));
-
-	list_del_init(&b->list);
-	mutex_unlock(&bc->lock);
 out:
 	b->flags		= 0;
 	b->written		= 0;
@@ -569,6 +590,14 @@ struct btree *bch2_btree_node_mem_alloc(struct bch_fs *c)
 	memalloc_nofs_restore(flags);
 	return b;
 err:
+	mutex_lock(&bc->lock);
+
+	if (b) {
+		list_add(&b->list, &bc->freed);
+		six_unlock_write(&b->c.lock);
+		six_unlock_intent(&b->c.lock);
+	}
+
 	/* Try to cannibalize another cached btree node: */
 	if (bc->alloc_lock == current) {
 		b = btree_node_cannibalize(c);