bcachefs: Don't BUG_ON() inode link count underflow

This switches that assertion to a bch2_trans_inconsistent() call, as it
should be.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fs-common.c

@@ -204,7 +204,9 @@ int bch2_link_trans(struct btree_trans *trans,
 		goto err;
 
 	inode_u->bi_ctime = now;
-	bch2_inode_nlink_inc(inode_u);
+	ret = bch2_inode_nlink_inc(inode_u);
+	if (ret)
+		return ret;
 
 	ret = bch2_inode_peek(trans, &dir_iter, dir_u, dir, BTREE_ITER_INTENT);
 	if (ret)
@@ -297,7 +299,7 @@ int bch2_unlink_trans(struct btree_trans *trans,
 		if (ret)
 			goto err;
 	} else {
-		bch2_inode_nlink_dec(inode_u);
+		bch2_inode_nlink_dec(trans, inode_u);
 	}
 
 	if (inode_u->bi_dir		== dirent_iter.pos.inode &&
@@ -462,7 +464,7 @@ int bch2_rename_trans(struct btree_trans *trans,
 	}
 
 	if (mode == BCH_RENAME_OVERWRITE)
-		bch2_inode_nlink_dec(dst_inode_u);
+		bch2_inode_nlink_dec(trans, dst_inode_u);
 
 	src_dir_u->bi_mtime		= now;
 	src_dir_u->bi_ctime		= now;

fs/bcachefs/inode.c

@@ -716,3 +716,36 @@ int bch2_inode_find_by_inum(struct bch_fs *c, subvol_inum inum,
 	return bch2_trans_do(c, NULL, NULL, 0,
 		bch2_inode_find_by_inum_trans(&trans, inum, inode));
 }
+
+int bch2_inode_nlink_inc(struct bch_inode_unpacked *bi)
+{
+	if (bi->bi_flags & BCH_INODE_UNLINKED)
+		bi->bi_flags &= ~BCH_INODE_UNLINKED;
+	else {
+		if (bi->bi_nlink == U32_MAX)
+			return -EINVAL;
+
+		bi->bi_nlink++;
+	}
+
+	return 0;
+}
+
+void bch2_inode_nlink_dec(struct btree_trans *trans, struct bch_inode_unpacked *bi)
+{
+	if (bi->bi_nlink && (bi->bi_flags & BCH_INODE_UNLINKED)) {
+		bch2_trans_inconsistent(trans, "inode %llu unlinked but link count nonzero",
+					bi->bi_inum);
+		return;
+	}
+
+	if (bi->bi_flags & BCH_INODE_UNLINKED) {
+		bch2_trans_inconsistent(trans, "inode %llu link count underflow", bi->bi_inum);
+		return;
+	}
+
+	if (bi->bi_nlink)
+		bi->bi_nlink--;
+	else
+		bi->bi_flags |= BCH_INODE_UNLINKED;
+}

fs/bcachefs/inode.h

@@ -164,23 +164,6 @@ static inline unsigned nlink_bias(umode_t mode)
 	return S_ISDIR(mode) ? 2 : 1;
 }
 
-static inline void bch2_inode_nlink_inc(struct bch_inode_unpacked *bi)
-{
-	if (bi->bi_flags & BCH_INODE_UNLINKED)
-		bi->bi_flags &= ~BCH_INODE_UNLINKED;
-	else
-		bi->bi_nlink++;
-}
-
-static inline void bch2_inode_nlink_dec(struct bch_inode_unpacked *bi)
-{
-	BUG_ON(bi->bi_flags & BCH_INODE_UNLINKED);
-	if (bi->bi_nlink)
-		bi->bi_nlink--;
-	else
-		bi->bi_flags |= BCH_INODE_UNLINKED;
-}
-
 static inline unsigned bch2_inode_nlink_get(struct bch_inode_unpacked *bi)
 {
 	return bi->bi_flags & BCH_INODE_UNLINKED
@@ -200,4 +183,7 @@ static inline void bch2_inode_nlink_set(struct bch_inode_unpacked *bi,
 	}
 }
 
+int bch2_inode_nlink_inc(struct bch_inode_unpacked *);
+void bch2_inode_nlink_dec(struct btree_trans *, struct bch_inode_unpacked *);
+
 #endif /* _BCACHEFS_INODE_H */