NFS Fault Injection

Fault injection is a method for forcing errors that may not normally occur, or may be difficult to reproduce. Forcing these errors in a controlled environment can help the developer find and fix bugs before their code is shipped in a production system. Injecting an error on the Linux NFS server will allow us to observe how the client reacts and if it manages to recover its state correctly.

NFSD_FAULT_INJECTION must be selected when configuring the kernel to use this feature.

Using Fault Injection

On the client, mount the fault injection server through NFS v4.0+ and do some work over NFS (open files, take locks, …).

On the server, mount the debugfs filesystem to <debug_dir> and ls <debug_dir>/nfsd. This will show a list of files that will be used for injecting faults on the NFS server. As root, write a number n to the file corresponding to the action you want the server to take. The server will then process the first n items it finds. So if you want to forget 5 locks, echo ‘5’ to <debug_dir>/nfsd/forget_locks. A value of 0 will tell the server to forget all corresponding items. A log message will be created containing the number of items forgotten (check dmesg).

Go back to work on the client and check if the client recovered from the error correctly.

Available Faults

The NFS server keeps a list of clients that have placed a mount call. If this list is cleared, the server will have no knowledge of who the client is, forcing the client to reauthenticate with the server.
The NFS server keeps a list of what files are currently opened and who they were opened by. Clearing this list will force the client to reopen its files.
The NFS server keeps a list of what files are currently locked in the VFS. Clearing this list will force the client to reclaim its locks (files are unlocked through the VFS as they are cleared from this list).
A delegation is used to assure the client that a file, or part of a file, has not changed since the delegation was awarded. Clearing this list will force the client to reacquire its delegation before accessing the file again.
Delegations can be recalled by the server when another client attempts to access a file. This test will notify the client that its delegation has been revoked, forcing the client to reacquire the delegation before using the file again.

tools/nfs/ script

This script has been created to ease the fault injection process. This script will detect the mounted debugfs directory and write to the files located there based on the arguments passed by the user. For example, running forget_locks 1 as root will instruct the server to forget one lock. Running inject_faults forget_locks will instruct the server to forgetall locks.