diff --git a/src/cpuset.rs b/src/cpuset.rs
index 55a50a9..1d0ed45 100644
--- a/src/cpuset.rs
+++ b/src/cpuset.rs
@@ -309,7 +309,7 @@ impl CpuSetController {
     ///
     /// Syntax is a comma separated list of CPUs, with an additional extension that ranges can
     /// be represented via dashes.
-    pub fn set_cpus(&self, cpus: &String) -> Result<()> {
+    pub fn set_cpus(&self, cpus: &str) -> Result<()> {
         self.open_path("cpuset.cpus", true).and_then(|mut file| {
             file.write_all(cpus.as_ref())
                 .map_err(|e| Error::with_cause(WriteFailed, e))
@@ -319,7 +319,7 @@ impl CpuSetController {
     /// Set the memory nodes that the tasks in this control group can use.
     ///
     /// Syntax is the same as with `set_cpus()`.
-    pub fn set_mems(&self, mems: &String) -> Result<()> {
+    pub fn set_mems(&self, mems: &str) -> Result<()> {
         self.open_path("cpuset.mems", true).and_then(|mut file| {
             file.write_all(mems.as_ref())
                 .map_err(|e| Error::with_cause(WriteFailed, e))
diff --git a/src/devices.rs b/src/devices.rs
index 355ea63..e3371f8 100644
--- a/src/devices.rs
+++ b/src/devices.rs
@@ -94,7 +94,7 @@ impl DevicePermissions {
     }
 
     /// Checks whether the string is a valid descriptor of DevicePermissions.
-    pub fn is_valid(s: &String) -> bool {
+    pub fn is_valid(s: &str) -> bool {
         if s == "" {
             return false;
         }
@@ -116,18 +116,18 @@ impl DevicePermissions {
     }
 
     /// Convert a string into DevicePermissions.
-    ///
-    /// NOTE: This function makes no effort in verifying the String.
-    pub fn from_string(s: &String) -> Vec<DevicePermissions> {
+    pub fn from_str(s: &str) -> Result<Vec<DevicePermissions>> {
         let mut v = Vec::new();
         if s == "" {
-            return v;
+            return Ok(v);
         }
         for e in s.chars() {
-            v.push(DevicePermissions::from_char(e).unwrap());
+            let perm = DevicePermissions::from_char(e)
+                .ok_or_else(|| Error::new(ParseError))?;
+            v.push(perm);
         }
 
-        v
+        Ok(v)
     }
 }
 
@@ -285,7 +285,7 @@ impl DevicesController {
                                          acc, ls, devtype, major, minor, &ls[3]);
                                 Err(Error::new(ParseError))
                             } else {
-                                let access = DevicePermissions::from_string(&ls[3]);
+                                let access = DevicePermissions::from_str(&ls[3])?;
                                 let mut acc = acc.unwrap();
                                 acc.push(DeviceResource {
                                     allow: true,
diff --git a/src/hugetlb.rs b/src/hugetlb.rs
index ee5cbf4..db714d1 100644
--- a/src/hugetlb.rs
+++ b/src/hugetlb.rs
@@ -94,34 +94,34 @@ impl HugeTlbController {
     }
 
     /// Whether the system supports `hugetlb_size` hugepages.
-    pub fn size_supported(&self, _hugetlb_size: String) -> bool {
+    pub fn size_supported(&self, _hugetlb_size: &str) -> bool {
         // TODO
         true
     }
 
     /// Check how many times has the limit of `hugetlb_size` hugepages been hit.
-    pub fn failcnt(&self, hugetlb_size: &String) -> Result<u64> {
+    pub fn failcnt(&self, hugetlb_size: &str) -> Result<u64> {
         self.open_path(&format!("hugetlb.{}.failcnt", hugetlb_size), false)
             .and_then(read_u64_from)
     }
 
     /// Get the limit (in bytes) of how much memory can be backed by hugepages of a certain size
     /// (`hugetlb_size`).
-    pub fn limit_in_bytes(&self, hugetlb_size: &String) -> Result<u64> {
+    pub fn limit_in_bytes(&self, hugetlb_size: &str) -> Result<u64> {
         self.open_path(&format!("hugetlb.{}.limit_in_bytes", hugetlb_size), false)
             .and_then(read_u64_from)
     }
 
     /// Get the current usage of memory that is backed by hugepages of a certain size
     /// (`hugetlb_size`).
-    pub fn usage_in_bytes(&self, hugetlb_size: &String) -> Result<u64> {
+    pub fn usage_in_bytes(&self, hugetlb_size: &str) -> Result<u64> {
         self.open_path(&format!("hugetlb.{}.usage_in_bytes", hugetlb_size), false)
             .and_then(read_u64_from)
     }
 
     /// Get the maximum observed usage of memory that is backed by hugepages of a certain size
     /// (`hugetlb_size`).
-    pub fn max_usage_in_bytes(&self, hugetlb_size: &String) -> Result<u64> {
+    pub fn max_usage_in_bytes(&self, hugetlb_size: &str) -> Result<u64> {
         self.open_path(
             &format!("hugetlb.{}.max_usage_in_bytes", hugetlb_size),
             false,
@@ -130,7 +130,7 @@ impl HugeTlbController {
 
     /// Set the limit (in bytes) of how much memory can be backed by hugepages of a certain size
     /// (`hugetlb_size`).
-    pub fn set_limit_in_bytes(&self, hugetlb_size: &String, limit: u64) -> Result<()> {
+    pub fn set_limit_in_bytes(&self, hugetlb_size: &str, limit: u64) -> Result<()> {
         self.open_path(&format!("hugetlb.{}.limit_in_bytes", hugetlb_size), true)
             .and_then(|mut file| {
                 file.write_all(limit.to_string().as_ref())
diff --git a/src/net_prio.rs b/src/net_prio.rs
index 4bd2672..9fcb428 100644
--- a/src/net_prio.rs
+++ b/src/net_prio.rs
@@ -133,7 +133,7 @@ impl NetPrioController {
     }
 
     /// Set the priority of the network traffic on `eif` to be `prio`.
-    pub fn set_if_prio(&self, eif: &String, prio: u64) -> Result<()> {
+    pub fn set_if_prio(&self, eif: &str, prio: u64) -> Result<()> {
         self.open_path("net_prio.ifpriomap", true)
             .and_then(|mut file| {
                 file.write_all(format!("{} {}", eif, prio).as_ref())
diff --git a/src/rdma.rs b/src/rdma.rs
index 5c8879e..ea4b078 100644
--- a/src/rdma.rs
+++ b/src/rdma.rs
@@ -86,7 +86,7 @@ impl RdmaController {
     }
 
     /// Set a maximum usage for each RDMA/IB resource.
-    pub fn set_max(&self, max: &String) -> Result<()> {
+    pub fn set_max(&self, max: &str) -> Result<()> {
         self.open_path("rdma.max", true).and_then(|mut file| {
             file.write_all(max.as_ref())
                 .map_err(|e| Error::with_cause(WriteFailed, e))