What Happens Inside the Linux Kernel When You Call the mount Command

When you use the mount command in a Linux system, a series of complex processes occur within the Linux kernel. This command initiates a process to attach a filesystem to a specific point in the directory tree. Here is a detailed breakdown of what happens:

1. User Space to Kernel Space Transition

The mount command is executed in user space. It calls the mount system call, which transitions control to the kernel. This transition is crucial as it moves the process into the kernel environment where it can interact with lower-level system resources.

2. System Call Handling

The kernel receives the system call and begins processing it. The relevant syscall handler, typically the do_mount function, is invoked. This handler is responsible for interpreting the mount command and coordinating the subsequent steps.

3. Parameter Validation

The kernel checks the parameters passed to the mount command, such as the device to be mounted, the mount point, the filesystem type, and mount options. This validation includes:

Validating the existence of the mount point. Checking permissions to ensure the user has the necessary rights to mount the filesystem.

4. Filesystem Type Identification

The kernel identifies the type of filesystem to be mounted. If a filesystem type is specified, it uses that information. Otherwise, it attempts to auto-detect the type based on the device's superblock.

5. Filesystem Initialization

The kernel calls the appropriate filesystem's mount function (e.g., ext4_mount, nfs_mount, etc.), which initializes the filesystem structure and prepares it for mounting.

6. Superblock Creation or Update

A superblock structure is created or updated. The superblock contains metadata about the filesystem, including its type, size, and state. This structure is crucial for maintaining filesystem information.

7. Device Access

The kernel accesses the specified device (e.g., a hard drive, USB drive) to read the filesystem's superblock and other necessary data. This step ensures that the kernel has the correct information to mount the filesystem properly.

8. Mounting Process

The filesystem is mounted by linking the superblock to the mount point in the kernel's internal data structures. This involves updating the virtual filesystem (VFS) layer, which is responsible for managing different filesystem types uniformly.

9. Updating Directory Structure

The kernel updates the directory structure to reflect the mount. The mount point now points to the root of the newly mounted filesystem, effectively integrating it into the directory tree.

10. Post-Mount Actions

Any specified mount options (e.g., read-only, no-utE) are applied. The kernel may also set up any necessary caching or buffers to ensure efficient access to the filesystem.

11. Return to User Space

Once the filesystem is successfully mounted, control returns to user space, and the mount command completes its execution. If something goes wrong during the process, the command returns an error code to the user.

Summary

The mount command triggers a series of operations within the Linux kernel. These operations involve validating parameters, initializing filesystem structures, accessing devices, and updating the virtual filesystem to make the new filesystem available at the specified mount point. Each of these steps is critical for ensuring that the filesystem is correctly integrated into the Linux operating system.