diff --git a/FileDeviceUtils.cpp b/FileDeviceUtils.cpp
new file mode 100644
index 0000000..bc9f4bd
--- /dev/null
+++ b/FileDeviceUtils.cpp
@@ -0,0 +1,115 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "FileDeviceUtils.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <linux/fs.h>
+#include <linux/fiemap.h>
+#include <mntent.h>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/unique_fd.h>
+
+namespace {
+
+std::unique_ptr<struct fiemap> alloc_fiemap(uint32_t extent_count);
+
+}
+
+namespace android {
+namespace vold {
+
+// Given a file path, look for the corresponding block device in /proc/mount
+std::string BlockDeviceForPath(const std::string &path)
+{
+    std::unique_ptr<FILE, int(*)(FILE*)> mnts(setmntent("/proc/mounts", "re"), endmntent);
+    if (!mnts) {
+        PLOG(ERROR) << "Unable to open /proc/mounts";
+        return "";
+    }
+    std::string result;
+    size_t best_length = 0;
+    struct mntent *mnt; // getmntent returns a thread local, so it's safe.
+    while ((mnt = getmntent(mnts.get())) != nullptr) {
+        auto l = strlen(mnt->mnt_dir);
+        if (l > best_length &&
+            path.size() > l &&
+            path[l] == '/' &&
+            path.compare(0, l, mnt->mnt_dir) == 0) {
+                result = mnt->mnt_fsname;
+                best_length = l;
+        }
+    }
+    if (result.empty()) {
+        LOG(ERROR) <<"Didn't find a mountpoint to match path " << path;
+        return "";
+    }
+    LOG(DEBUG) << "For path " << path << " block device is " << result;
+    return result;
+}
+
+std::unique_ptr<struct fiemap> PathFiemap(const std::string &path, uint32_t extent_count)
+{
+    android::base::unique_fd fd(TEMP_FAILURE_RETRY(open(
+        path.c_str(), O_RDONLY | O_CLOEXEC, 0)));
+    if (fd == -1) {
+        if (errno == ENOENT) {
+            PLOG(DEBUG) << "Unable to open " << path;
+        } else {
+            PLOG(ERROR) << "Unable to open " << path;
+        }
+        return nullptr;
+    }
+    auto fiemap = alloc_fiemap(extent_count);
+    if (ioctl(fd.get(), FS_IOC_FIEMAP, fiemap.get()) != 0) {
+        PLOG(ERROR) << "Unable to FIEMAP " << path;
+        return nullptr;
+    }
+    auto mapped = fiemap->fm_mapped_extents;
+    if (mapped < 1 || mapped > extent_count) {
+        LOG(ERROR) << "Extent count not in bounds 1 <= " << mapped << " <= " << extent_count
+            << " in " << path;
+        return nullptr;
+    }
+    return fiemap;
+}
+
+}  // namespace vold
+}  // namespace android
+
+namespace {
+
+std::unique_ptr<struct fiemap> alloc_fiemap(uint32_t extent_count)
+{
+    size_t allocsize = offsetof(struct fiemap, fm_extents[extent_count]);
+    std::unique_ptr<struct fiemap> res(new (::operator new (allocsize)) struct fiemap);
+    memset(res.get(), 0, allocsize);
+    res->fm_start = 0;
+    res->fm_length = UINT64_MAX;
+    res->fm_flags = 0;
+    res->fm_extent_count = extent_count;
+    res->fm_mapped_extents = 0;
+    return res;
+}
+
+}