bcachefs: Fix reattach_inode() for snapshots

reattach_inode() was broken w.r.t. snapshots - we'd lookup the subvolume
to look up lost+found, but if we're in an interior node snapshot that
didn't make any sense.

Instead, this adds a dirent path for creating in a specific snapshot,
skipping the subvolume; and we also make sure to create lost+found in
the root snapshot, to avoid conflicts with lost+found being created in
overlapping snapshots.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/dirent.c

@@ -198,6 +198,34 @@ static struct bkey_i_dirent *dirent_create_key(struct btree_trans *trans,
 	return dirent;
 }
 
+int bch2_dirent_create_snapshot(struct btree_trans *trans,
+			u64 dir, u32 snapshot,
+			const struct bch_hash_info *hash_info,
+			u8 type, const struct qstr *name, u64 dst_inum,
+			u64 *dir_offset,
+			bch_str_hash_flags_t str_hash_flags)
+{
+	subvol_inum zero_inum = { 0 };
+	struct bkey_i_dirent *dirent;
+	int ret;
+
+	dirent = dirent_create_key(trans, zero_inum, type, name, dst_inum);
+	ret = PTR_ERR_OR_ZERO(dirent);
+	if (ret)
+		return ret;
+
+	dirent->k.p.inode	= dir;
+	dirent->k.p.snapshot	= snapshot;
+
+	ret = bch2_hash_set_snapshot(trans, bch2_dirent_hash_desc, hash_info,
+				     zero_inum, snapshot,
+				     &dirent->k_i, str_hash_flags,
+				     BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
+	*dir_offset = dirent->k.p.offset;
+
+	return ret;
+}
+
 int bch2_dirent_create(struct btree_trans *trans, subvol_inum dir,
 		       const struct bch_hash_info *hash_info,
 		       u8 type, const struct qstr *name, u64 dst_inum,

fs/bcachefs/dirent.h

@@ -35,6 +35,10 @@ static inline unsigned dirent_val_u64s(unsigned len)
 int bch2_dirent_read_target(struct btree_trans *, subvol_inum,
 			    struct bkey_s_c_dirent, subvol_inum *);
 
+int bch2_dirent_create_snapshot(struct btree_trans *, u64, u32,
+			const struct bch_hash_info *, u8,
+			const struct qstr *, u64, u64 *,
+			bch_str_hash_flags_t);
 int bch2_dirent_create(struct btree_trans *, subvol_inum,
 		       const struct bch_hash_info *, u8,
 		       const struct qstr *, u64, u64 *,

fs/bcachefs/fsck.c

@@ -59,21 +59,6 @@ static s64 bch2_count_subdirs(struct btree_trans *trans, u64 inum,
 	return ret ?: subdirs;
 }
 
-static int __snapshot_lookup_subvol(struct btree_trans *trans, u32 snapshot,
-				    u32 *subvol)
-{
-	struct bch_snapshot s;
-	int ret = bch2_bkey_get_val_typed(trans, BTREE_ID_snapshots,
-					  POS(0, snapshot), 0,
-					  snapshot, &s);
-	if (!ret)
-		*subvol = le32_to_cpu(s.subvol);
-	else if (bch2_err_matches(ret, ENOENT))
-		bch_err(trans->c, "snapshot %u not found", snapshot);
-	return ret;
-
-}
-
 static int __subvol_lookup(struct btree_trans *trans, u32 subvol,
 			   u32 *snapshot, u64 *inum)
 {
@@ -227,35 +212,42 @@ static int __remove_dirent(struct btree_trans *trans, struct bpos pos)
 }
 
 /* Get lost+found, create if it doesn't exist: */
-static int lookup_lostfound(struct btree_trans *trans, u32 subvol,
+static int lookup_lostfound(struct btree_trans *trans, u32 snapshot,
 			    struct bch_inode_unpacked *lostfound)
 {
 	struct bch_fs *c = trans->c;
-	struct bch_inode_unpacked root;
-	struct bch_hash_info root_hash_info;
 	struct qstr lostfound_str = QSTR("lost+found");
-	subvol_inum root_inum = { .subvol = subvol };
 	u64 inum = 0;
 	unsigned d_type = 0;
-	u32 snapshot;
 	int ret;
 
-	ret = __subvol_lookup(trans, subvol, &snapshot, &root_inum.inum);
+	struct bch_snapshot_tree st;
+	ret = bch2_snapshot_tree_lookup(trans,
+			bch2_snapshot_tree(c, snapshot), &st);
 	if (ret)
 		return ret;
 
-	ret = __lookup_inode(trans, root_inum.inum, &root, &snapshot);
+	subvol_inum root_inum = { .subvol = le32_to_cpu(st.master_subvol) };
+	u32 subvol_snapshot;
+
+	ret = __subvol_lookup(trans, le32_to_cpu(st.master_subvol),
+			      &subvol_snapshot, &root_inum.inum);
+	bch_err_msg(c, ret, "looking up root subvol");
+	if (ret)
+		return ret;
+
+	struct bch_inode_unpacked root_inode;
+	struct bch_hash_info root_hash_info;
+	ret = __lookup_inode(trans, root_inum.inum, &root_inode, &snapshot);
 	if (ret)
 		return ret;
 
-	root_hash_info = bch2_hash_info_init(c, &root);
+	root_hash_info = bch2_hash_info_init(c, &root_inode);
 
 	ret = __lookup_dirent(trans, root_hash_info, root_inum,
 			      &lostfound_str, &inum, &d_type);
-	if (bch2_err_matches(ret, ENOENT)) {
-		bch_notice(c, "creating lost+found");
+	if (bch2_err_matches(ret, ENOENT))
 		goto create_lostfound;
-	}
 
 	bch_err_fn(c, ret);
 	if (ret)
@@ -273,13 +265,43 @@ static int lookup_lostfound(struct btree_trans *trans, u32 subvol,
 	return __lookup_inode(trans, inum, lostfound, &snapshot);
 
 create_lostfound:
+	/*
+	 * XXX: we could have a nicer log message here  if we had a nice way to
+	 * walk backpointers to print a path
+	 */
+	bch_notice(c, "creating lost+found in snapshot %u", le32_to_cpu(st.root_snapshot));
+
+	u64 now = bch2_current_time(c);
+	struct btree_iter lostfound_iter = { NULL };
+	u64 cpu = raw_smp_processor_id();
+
 	bch2_inode_init_early(c, lostfound);
+	bch2_inode_init_late(lostfound, now, 0, 0, S_IFDIR|0700, 0, &root_inode);
+	lostfound->bi_dir = root_inode.bi_inum;
+
+	root_inode.bi_nlink++;
+
+	ret = bch2_inode_create(trans, &lostfound_iter, lostfound, snapshot, cpu);
+	if (ret)
+		goto err;
 
-	ret = bch2_create_trans(trans, root_inum, &root,
-				lostfound, &lostfound_str,
-				0, 0, S_IFDIR|0700, 0, NULL, NULL,
-				(subvol_inum) { }, 0);
+	bch2_btree_iter_set_snapshot(&lostfound_iter, snapshot);
+	ret = bch2_btree_iter_traverse(&lostfound_iter);
+	if (ret)
+		goto err;
+
+	ret =   bch2_dirent_create_snapshot(trans,
+				root_inode.bi_inum, snapshot, &root_hash_info,
+				mode_to_type(lostfound->bi_mode),
+				&lostfound_str,
+				lostfound->bi_inum,
+				&lostfound->bi_dir_offset,
+				BCH_HASH_SET_MUST_CREATE) ?:
+		bch2_inode_write_flags(trans, &lostfound_iter, lostfound,
+				       BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
+err:
 	bch_err_msg(c, ret, "creating lost+found");
+	bch2_trans_iter_exit(trans, &lostfound_iter);
 	return ret;
 }
 
@@ -292,14 +314,9 @@ static int __reattach_inode(struct btree_trans *trans,
 	char name_buf[20];
 	struct qstr name;
 	u64 dir_offset = 0;
-	u32 subvol;
 	int ret;
 
-	ret = __snapshot_lookup_subvol(trans, inode_snapshot, &subvol);
-	if (ret)
-		return ret;
-
-	ret = lookup_lostfound(trans, subvol, &lostfound);
+	ret = lookup_lostfound(trans, inode_snapshot, &lostfound);
 	if (ret)
 		return ret;
 
@@ -316,11 +333,8 @@ static int __reattach_inode(struct btree_trans *trans,
 	snprintf(name_buf, sizeof(name_buf), "%llu", inode->bi_inum);
 	name = (struct qstr) QSTR(name_buf);
 
-	ret = bch2_dirent_create(trans,
-				 (subvol_inum) {
-					.subvol = subvol,
-					.inum = lostfound.bi_inum,
-				 },
+	ret = bch2_dirent_create_snapshot(trans,
+				lostfound.bi_inum, inode_snapshot,
 				&dir_hash,
 				inode_d_type(inode),
 				&name, inode->bi_inum, &dir_offset,