Configure read ahead for fuse mounts

For fuse read ahead can be configured by writing a value to the
/sys/class/bdi/{MAJOR}:{MINOR}/read_ahead_kb file.

There are several different ways of getting {MAJOR}:{MINOR} values of
the filesystem:

* Look at st_dev of stat("/mnt/user/0/emulated").
* Parse /proc/self/mountinfo.

Stat'ing approach is used since it's easier to implement.

Bug: 157982297
Test: atest vold_tests
Test: adb shell cat /proc/self/mountinfo to get MAJOR:MINOR
Test: adb shell cat /sys/class/bdi/{MAJOR}:{MINOR}/read_ahead_kb
Test: created public volume, checked it's read_ahead_kb is also 256
Change-Id: Id0c149c4af1ceabf3afc33b4100563a512b38316

Utils.cpp

@@ -63,6 +63,7 @@ using android::base::EndsWith;
 using android::base::ReadFileToString;
 using android::base::StartsWith;
 using android::base::StringPrintf;
+using android::base::unique_fd;
 
 namespace android {
 namespace vold {
@@ -92,6 +93,10 @@ static const char* kMediaProviderAppCtx = "u:r:mediaprovider_app:";
 // other between multiple threads.
 static std::mutex kSecurityLock;
 
+std::string GetFuseMountPathForUser(userid_t user_id, const std::string& relative_upper_path) {
+    return StringPrintf("/mnt/user/%d/%s", user_id, relative_upper_path.c_str());
+}
+
 status_t CreateDeviceNode(const std::string& path, dev_t dev) {
     std::lock_guard<std::mutex> lock(kSecurityLock);
     const char* cpath = path.c_str();
@@ -1355,6 +1360,33 @@ status_t EnsureDirExists(const std::string& path, mode_t mode, uid_t uid, gid_t
     return OK;
 }
 
+// Configures read ahead property of the fuse filesystem with the mount point |fuse_mount| by
+// writing |read_ahead_kb| to the /sys/class/bdi/MAJOR:MINOR/read_ahead_kb.
+void ConfigureReadAheadForFuse(const std::string& fuse_mount, size_t read_ahead_kb) {
+    LOG(INFO) << "Configuring read_ahead of " << fuse_mount << " fuse filesystem to "
+              << read_ahead_kb << "kb";
+    // First figure out MAJOR:MINOR of fuse_mount. Simplest way is to stat the path.
+    struct stat info;
+    if (stat(fuse_mount.c_str(), &info) != 0) {
+        PLOG(ERROR) << "Failed to stat " << fuse_mount;
+        return;
+    }
+    unsigned int maj = major(info.st_dev);
+    unsigned int min = minor(info.st_dev);
+    LOG(INFO) << fuse_mount << " has major:minor " << maj << ":" << min;
+    // We found major:minor of our filesystem, time to configure read ahead!
+    std::string read_ahead_file = StringPrintf("/sys/class/bdi/%u:%u/read_ahead_kb", maj, min);
+    unique_fd fd(TEMP_FAILURE_RETRY(open(read_ahead_file.c_str(), O_WRONLY | O_CLOEXEC)));
+    if (fd.get() == -1) {
+        PLOG(ERROR) << "Failed to open " << read_ahead_file;
+        return;
+    }
+    LOG(INFO) << "Writing " << read_ahead_kb << " to " << read_ahead_file;
+    if (!WriteStringToFd(std::to_string(read_ahead_kb), fd)) {
+        PLOG(ERROR) << "Failed to write to " << read_ahead_file;
+    }
+}
+
 status_t MountUserFuse(userid_t user_id, const std::string& absolute_lower_path,
                        const std::string& relative_upper_path, android::base::unique_fd* fuse_fd) {
     std::string pre_fuse_path(StringPrintf("/mnt/user/%d", user_id));

Utils.h

@@ -47,6 +47,8 @@ extern security_context_t sFsckUntrustedContext;
 // TODO remove this with better solution, b/64143519
 extern bool sSleepOnUnmount;
 
+std::string GetFuseMountPathForUser(userid_t user_id, const std::string& relative_upper_path);
+
 status_t CreateDeviceNode(const std::string& path, dev_t dev);
 status_t DestroyDeviceNode(const std::string& path);
 
@@ -174,6 +176,8 @@ bool FsyncDirectory(const std::string& dirname);
 
 bool writeStringToFile(const std::string& payload, const std::string& filename);
 
+void ConfigureReadAheadForFuse(const std::string& fuse_mount, size_t read_ahead_kb);
+
 status_t MountUserFuse(userid_t user_id, const std::string& absolute_lower_path,
                        const std::string& relative_upper_path, android::base::unique_fd* fuse_fd);
 

model/EmulatedVolume.cpp

@@ -402,6 +402,8 @@ status_t EmulatedVolume::doMount() {
             return res;
         }
 
+        ConfigureReadAheadForFuse(GetFuseMountPathForUser(user_id, label), 256u);
+
         // All mounts where successful, disable scope guards
         sdcardfs_guard.Disable();
         fuse_guard.Disable();

model/PublicVolume.cpp

@@ -250,6 +250,8 @@ status_t PublicVolume::doMount() {
                 return -EIO;
             }
         }
+
+        ConfigureReadAheadForFuse(GetFuseMountPathForUser(user_id, stableName), 256u);
     }
 
     return OK;