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One of the core features of scoutfs is the ability to transparently migrate file contents to and from an archive tier. For this to be transparent we need file system operations to trigger staging the file contents back into the file system as needed. This adds the infrastructure which operations use to wait for offline extents to come online and which provides userspace with a list of blocks that the operations are waiting for. We add some waiting infrastructure that callers use to lock, check for offline extents, and unlock and wait before checking again to see if they're still offline. We add these checks and waiting to data io operations that could encounter offline extents. This has to be done carefully so that we don't wait while holding locks that would prevent staging. We use per-task structures to discover when we are the first user of a cluster lock on an inode, indicating that it's safe for us to wait because we don't hold any locks. And while we're waiting our operation is tracked and reported to userspace through an ioctl. This is a non-blocking ioctl, it's up to userspace to decide how often to check and how large a region to stage. Waiters are woken up when the file contents could have changed, not specifically when we know that the extent has come online. This lets us wake waiters when their lock is revoked so that they can block waiting to reacquire the lock and test the extents again. It lets us provide coherent demand staging across the cluster without fine grained waiting protocols sent betwen the nodes. It may result in some spurious wakeups and work but hopefully it won't, and it's a very simple and functional first pass. Signed-off-by: Zach Brown <zab@versity.com>