1=========== 2NFS LOCALIO 3=========== 4 5Overview 6======== 7 8The LOCALIO auxiliary RPC protocol allows the Linux NFS client and 9server to reliably handshake to determine if they are on the same 10host. Select "NFS client and server support for LOCALIO auxiliary 11protocol" in menuconfig to enable CONFIG_NFS_LOCALIO in the kernel 12config (both CONFIG_NFS_FS and CONFIG_NFSD must also be enabled). 13 14Once an NFS client and server handshake as "local", the client will 15bypass the network RPC protocol for read, write and commit operations. 16Due to this XDR and RPC bypass, these operations will operate faster. 17 18The LOCALIO auxiliary protocol's implementation, which uses the same 19connection as NFS traffic, follows the pattern established by the NFS 20ACL protocol extension. 21 22The LOCALIO auxiliary protocol is needed to allow robust discovery of 23clients local to their servers. In a private implementation that 24preceded use of this LOCALIO protocol, a fragile sockaddr network 25address based match against all local network interfaces was attempted. 26But unlike the LOCALIO protocol, the sockaddr-based matching didn't 27handle use of iptables or containers. 28 29The robust handshake between local client and server is just the 30beginning, the ultimate use case this locality makes possible is the 31client is able to open files and issue reads, writes and commits 32directly to the server without having to go over the network. The 33requirement is to perform these loopback NFS operations as efficiently 34as possible, this is particularly useful for container use cases 35(e.g. kubernetes) where it is possible to run an IO job local to the 36server. 37 38The performance advantage realized from LOCALIO's ability to bypass 39using XDR and RPC for reads, writes and commits can be extreme, e.g.: 40 41fio for 20 secs with directio, qd of 8, 16 libaio threads: 42 - With LOCALIO: 43 4K read: IOPS=979k, BW=3825MiB/s (4011MB/s)(74.7GiB/20002msec) 44 4K write: IOPS=165k, BW=646MiB/s (678MB/s)(12.6GiB/20002msec) 45 128K read: IOPS=402k, BW=49.1GiB/s (52.7GB/s)(982GiB/20002msec) 46 128K write: IOPS=11.5k, BW=1433MiB/s (1503MB/s)(28.0GiB/20004msec) 47 48 - Without LOCALIO: 49 4K read: IOPS=79.2k, BW=309MiB/s (324MB/s)(6188MiB/20003msec) 50 4K write: IOPS=59.8k, BW=234MiB/s (245MB/s)(4671MiB/20002msec) 51 128K read: IOPS=33.9k, BW=4234MiB/s (4440MB/s)(82.7GiB/20004msec) 52 128K write: IOPS=11.5k, BW=1434MiB/s (1504MB/s)(28.0GiB/20011msec) 53 54fio for 20 secs with directio, qd of 8, 1 libaio thread: 55 - With LOCALIO: 56 4K read: IOPS=230k, BW=898MiB/s (941MB/s)(17.5GiB/20001msec) 57 4K write: IOPS=22.6k, BW=88.3MiB/s (92.6MB/s)(1766MiB/20001msec) 58 128K read: IOPS=38.8k, BW=4855MiB/s (5091MB/s)(94.8GiB/20001msec) 59 128K write: IOPS=11.4k, BW=1428MiB/s (1497MB/s)(27.9GiB/20001msec) 60 61 - Without LOCALIO: 62 4K read: IOPS=77.1k, BW=301MiB/s (316MB/s)(6022MiB/20001msec) 63 4K write: IOPS=32.8k, BW=128MiB/s (135MB/s)(2566MiB/20001msec) 64 128K read: IOPS=24.4k, BW=3050MiB/s (3198MB/s)(59.6GiB/20001msec) 65 128K write: IOPS=11.4k, BW=1430MiB/s (1500MB/s)(27.9GiB/20001msec) 66 67FAQ 68=== 69 701. What are the use cases for LOCALIO? 71 72 a. Workloads where the NFS client and server are on the same host 73 realize improved IO performance. In particular, it is common when 74 running containerised workloads for jobs to find themselves 75 running on the same host as the knfsd server being used for 76 storage. 77 782. What are the requirements for LOCALIO? 79 80 a. Bypass use of the network RPC protocol as much as possible. This 81 includes bypassing XDR and RPC for open, read, write and commit 82 operations. 83 b. Allow client and server to autonomously discover if they are 84 running local to each other without making any assumptions about 85 the local network topology. 86 c. Support the use of containers by being compatible with relevant 87 namespaces (e.g. network, user, mount). 88 d. Support all versions of NFS. NFSv3 is of particular importance 89 because it has wide enterprise usage and pNFS flexfiles makes use 90 of it for the data path. 91 923. Why doesn’t LOCALIO just compare IP addresses or hostnames when 93 deciding if the NFS client and server are co-located on the same 94 host? 95 96 Since one of the main use cases is containerised workloads, we cannot 97 assume that IP addresses will be shared between the client and 98 server. This sets up a requirement for a handshake protocol that 99 needs to go over the same connection as the NFS traffic in order to 100 identify that the client and the server really are running on the 101 same host. The handshake uses a secret that is sent over the wire, 102 and can be verified by both parties by comparing with a value stored 103 in shared kernel memory if they are truly co-located. 104 1054. Does LOCALIO improve pNFS flexfiles? 106 107 Yes, LOCALIO complements pNFS flexfiles by allowing it to take 108 advantage of NFS client and server locality. Policy that initiates 109 client IO as closely to the server where the data is stored naturally 110 benefits from the data path optimization LOCALIO provides. 111 1125. Why not develop a new pNFS layout to enable LOCALIO? 113 114 A new pNFS layout could be developed, but doing so would put the 115 onus on the server to somehow discover that the client is co-located 116 when deciding to hand out the layout. 117 There is value in a simpler approach (as provided by LOCALIO) that 118 allows the NFS client to negotiate and leverage locality without 119 requiring more elaborate modeling and discovery of such locality in a 120 more centralized manner. 121 1226. Why is having the client perform a server-side file OPEN, without 123 using RPC, beneficial? Is the benefit pNFS specific? 124 125 Avoiding the use of XDR and RPC for file opens is beneficial to 126 performance regardless of whether pNFS is used. Especially when 127 dealing with small files its best to avoid going over the wire 128 whenever possible, otherwise it could reduce or even negate the 129 benefits of avoiding the wire for doing the small file I/O itself. 130 Given LOCALIO's requirements the current approach of having the 131 client perform a server-side file open, without using RPC, is ideal. 132 If in the future requirements change then we can adapt accordingly. 133 1347. Why is LOCALIO only supported with UNIX Authentication (AUTH_UNIX)? 135 136 Strong authentication is usually tied to the connection itself. It 137 works by establishing a context that is cached by the server, and 138 that acts as the key for discovering the authorisation token, which 139 can then be passed to rpc.mountd to complete the authentication 140 process. On the other hand, in the case of AUTH_UNIX, the credential 141 that was passed over the wire is used directly as the key in the 142 upcall to rpc.mountd. This simplifies the authentication process, and 143 so makes AUTH_UNIX easier to support. 144 1458. How do export options that translate RPC user IDs behave for LOCALIO 146 operations (eg. root_squash, all_squash)? 147 148 Export options that translate user IDs are managed by nfsd_setuser() 149 which is called by nfsd_setuser_and_check_port() which is called by 150 __fh_verify(). So they get handled exactly the same way for LOCALIO 151 as they do for non-LOCALIO. 152 1539. How does LOCALIO make certain that object lifetimes are managed 154 properly given NFSD and NFS operate in different contexts? 155 156 See the detailed "NFS Client and Server Interlock" section below. 157 158RPC 159=== 160 161The LOCALIO auxiliary RPC protocol consists of a single "UUID_IS_LOCAL" 162RPC method that allows the Linux NFS client to verify the local Linux 163NFS server can see the nonce (single-use UUID) the client generated and 164made available in nfs_common. This protocol isn't part of an IETF 165standard, nor does it need to be considering it is Linux-to-Linux 166auxiliary RPC protocol that amounts to an implementation detail. 167 168The UUID_IS_LOCAL method encodes the client generated uuid_t in terms of 169the fixed UUID_SIZE (16 bytes). The fixed size opaque encode and decode 170XDR methods are used instead of the less efficient variable sized 171methods. 172 173The RPC program number for the NFS_LOCALIO_PROGRAM is 400122 (as assigned 174by IANA, see https://www.iana.org/assignments/rpc-program-numbers/ ): 175Linux Kernel Organization 400122 nfslocalio 176 177The LOCALIO protocol spec in rpcgen syntax is:: 178 179 /* raw RFC 9562 UUID */ 180 #define UUID_SIZE 16 181 typedef u8 uuid_t<UUID_SIZE>; 182 183 program NFS_LOCALIO_PROGRAM { 184 version LOCALIO_V1 { 185 void 186 NULL(void) = 0; 187 188 void 189 UUID_IS_LOCAL(uuid_t) = 1; 190 } = 1; 191 } = 400122; 192 193LOCALIO uses the same transport connection as NFS traffic. As such, 194LOCALIO is not registered with rpcbind. 195 196NFS Common and Client/Server Handshake 197====================================== 198 199fs/nfs_common/nfslocalio.c provides interfaces that enable an NFS client 200to generate a nonce (single-use UUID) and associated short-lived 201nfs_uuid_t struct, register it with nfs_common for subsequent lookup and 202verification by the NFS server and if matched the NFS server populates 203members in the nfs_uuid_t struct. The NFS client then uses nfs_common to 204transfer the nfs_uuid_t from its nfs_uuids to the nn->nfsd_serv 205clients_list from the nfs_common's uuids_list. See: 206fs/nfs/localio.c:nfs_local_probe() 207 208nfs_common's nfs_uuids list is the basis for LOCALIO enablement, as such 209it has members that point to nfsd memory for direct use by the client 210(e.g. 'net' is the server's network namespace, through it the client can 211access nn->nfsd_serv with proper rcu read access). It is this client 212and server synchronization that enables advanced usage and lifetime of 213objects to span from the host kernel's nfsd to per-container knfsd 214instances that are connected to nfs client's running on the same local 215host. 216 217NFS Client and Server Interlock 218=============================== 219 220LOCALIO provides the nfs_uuid_t object and associated interfaces to 221allow proper network namespace (net-ns) and NFSD object refcounting: 222 223 We don't want to keep a long-term counted reference on each NFSD's 224 net-ns in the client because that prevents a server container from 225 completely shutting down. 226 227 So we avoid taking a reference at all and rely on the per-cpu 228 reference to the server (detailed below) being sufficient to keep 229 the net-ns active. This involves allowing the NFSD's net-ns exit 230 code to iterate all active clients and clear their ->net pointers 231 (which are needed to find the per-cpu-refcount for the nfsd_serv). 232 233 Details: 234 235 - Embed nfs_uuid_t in nfs_client. nfs_uuid_t provides a list_head 236 that can be used to find the client. It does add the 16-byte 237 uuid_t to nfs_client so it is bigger than needed (given that 238 uuid_t is only used during the initial NFS client and server 239 LOCALIO handshake to determine if they are local to each other). 240 If that is really a problem we can find a fix. 241 242 - When the nfs server confirms that the uuid_t is local, it moves 243 the nfs_uuid_t onto a per-net-ns list in NFSD's nfsd_net. 244 245 - When each server's net-ns is shutting down - in a "pre_exit" 246 handler, all these nfs_uuid_t have their ->net cleared. There is 247 an rcu_synchronize() call between pre_exit() handlers and exit() 248 handlers so any caller that sees nfs_uuid_t ->net as not NULL can 249 safely manage the per-cpu-refcount for nfsd_serv. 250 251 - The client's nfs_uuid_t is passed to nfsd_open_local_fh() so it 252 can safely dereference ->net in a private rcu_read_lock() section 253 to allow safe access to the associated nfsd_net and nfsd_serv. 254 255So LOCALIO required the introduction and use of NFSD's percpu_ref to 256interlock nfsd_destroy_serv() and nfsd_open_local_fh(), to ensure each 257nn->nfsd_serv is not destroyed while in use by nfsd_open_local_fh(), and 258warrants a more detailed explanation: 259 260 nfsd_open_local_fh() uses nfsd_serv_try_get() before opening its 261 nfsd_file handle and then the caller (NFS client) must drop the 262 reference for the nfsd_file and associated nn->nfsd_serv using 263 nfs_file_put_local() once it has completed its IO. 264 265 This interlock working relies heavily on nfsd_open_local_fh() being 266 afforded the ability to safely deal with the possibility that the 267 NFSD's net-ns (and nfsd_net by association) may have been destroyed 268 by nfsd_destroy_serv() via nfsd_shutdown_net() -- which is only 269 possible given the nfs_uuid_t ->net pointer managemenet detailed 270 above. 271 272All told, this elaborate interlock of the NFS client and server has been 273verified to fix an easy to hit crash that would occur if an NFSD 274instance running in a container, with a LOCALIO client mounted, is 275shutdown. Upon restart of the container and associated NFSD the client 276would go on to crash due to NULL pointer dereference that occurred due 277to the LOCALIO client's attempting to nfsd_open_local_fh(), using 278nn->nfsd_serv, without having a proper reference on nn->nfsd_serv. 279 280NFS Client issues IO instead of Server 281====================================== 282 283Because LOCALIO is focused on protocol bypass to achieve improved IO 284performance, alternatives to the traditional NFS wire protocol (SUNRPC 285with XDR) must be provided to access the backing filesystem. 286 287See fs/nfs/localio.c:nfs_local_open_fh() and 288fs/nfsd/localio.c:nfsd_open_local_fh() for the interface that makes 289focused use of select nfs server objects to allow a client local to a 290server to open a file pointer without needing to go over the network. 291 292The client's fs/nfs/localio.c:nfs_local_open_fh() will call into the 293server's fs/nfsd/localio.c:nfsd_open_local_fh() and carefully access 294both the associated nfsd network namespace and nn->nfsd_serv in terms of 295RCU. If nfsd_open_local_fh() finds that the client no longer sees valid 296nfsd objects (be it struct net or nn->nfsd_serv) it returns -ENXIO 297to nfs_local_open_fh() and the client will try to reestablish the 298LOCALIO resources needed by calling nfs_local_probe() again. This 299recovery is needed if/when an nfsd instance running in a container were 300to reboot while a LOCALIO client is connected to it. 301 302Once the client has an open nfsd_file pointer it will issue reads, 303writes and commits directly to the underlying local filesystem (normally 304done by the nfs server). As such, for these operations, the NFS client 305is issuing IO to the underlying local filesystem that it is sharing with 306the NFS server. See: fs/nfs/localio.c:nfs_local_doio() and 307fs/nfs/localio.c:nfs_local_commit(). 308 309Security 310======== 311 312Localio is only supported when UNIX-style authentication (AUTH_UNIX, aka 313AUTH_SYS) is used. 314 315Care is taken to ensure the same NFS security mechanisms are used 316(authentication, etc) regardless of whether LOCALIO or regular NFS 317access is used. The auth_domain established as part of the traditional 318NFS client access to the NFS server is also used for LOCALIO. 319 320Relative to containers, LOCALIO gives the client access to the network 321namespace the server has. This is required to allow the client to access 322the server's per-namespace nfsd_net struct. With traditional NFS, the 323client is afforded this same level of access (albeit in terms of the NFS 324protocol via SUNRPC). No other namespaces (user, mount, etc) have been 325altered or purposely extended from the server to the client. 326 327Testing 328======= 329 330The LOCALIO auxiliary protocol and associated NFS LOCALIO read, write 331and commit access have proven stable against various test scenarios: 332 333- Client and server both on the same host. 334 335- All permutations of client and server support enablement for both 336 local and remote client and server. 337 338- Testing against NFS storage products that don't support the LOCALIO 339 protocol was also performed. 340 341- Client on host, server within a container (for both v3 and v4.2). 342 The container testing was in terms of podman managed containers and 343 includes successful container stop/restart scenario. 344 345- Formalizing these test scenarios in terms of existing test 346 infrastructure is on-going. Initial regular coverage is provided in 347 terms of ktest running xfstests against a LOCALIO-enabled NFS loopback 348 mount configuration, and includes lockdep and KASAN coverage, see: 349 https://evilpiepirate.org/~testdashboard/ci?user=snitzer&branch=snitm-nfs-next 350 https://github.com/koverstreet/ktest 351 352- Various kdevops testing (in terms of "Chuck's BuildBot") has been 353 performed to regularly verify the LOCALIO changes haven't caused any 354 regressions to non-LOCALIO NFS use cases. 355 356- All of Hammerspace's various sanity tests pass with LOCALIO enabled 357 (this includes numerous pNFS and flexfiles tests). 358