nfsd: implement server-stats-get netlink handler

Implement nfsd_nl_server_stats_get_dumpit() which exposes the NFS server statistics currently available via /proc/net/rpc/nfsd through the nfsd generic netlink family. The handler uses a dump operation to stream statistics across one or more netlink messages. The reply is divided into sections that are emitted in order: - scalar stats (reply cache, filehandle, IO, network, RPC), emitted once in the first message, then - per-version procedure counts (proc2/3/4-ops) and the NFSv4 per-operation counts (proc4ops-ops), using the per-netns vs_count arrays. cb->args[0] tracks the current section and cb->args[1] the entry index within it, so a section that does not fit in the current message is closed and resumed in the next one. This matters because the first dump message is allocated at NLMSG_GOODSIZE (a single page on most architectures) regardless of the client's receive buffer; packing every counter into one message would overflow it and fail the dump with -EMSGSIZE. Userspace merges the attributes from every message. This allows nfsstat to retrieve server statistics via netlink with a procfs fallback for older kernels.

JLJeff Layton committed 4 months ago