嵌入式linux开发，使用nfs挂载根文件系统时，内核无挂载文件系统，报错如下：
VFS: Unable to mount root fs via NFS, trying floppy. VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6 Please append a correct "root=" boot option; here are the available partitions: 1f00 2048 mtdblock0 (driver?) 1f01 20480 mtdblock1 (driver?) 1f02 239616 mtdblock2 (driver?) Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0) CPU: 0 PID: 1 Comm: swapper Not tainted 3.10.101 #233 Backtrace: [二、问题分析。] (dump_backtrace+0x0/0x10c) from [ ] (show_stack+0x18/0x1c) r6:c05b3d68 r5:00008000 r4:c051a658 r3:00000000 [ ] (show_stack+0x0/0x1c) from [ ] (dump_stack+0x20/0x28) [ ] (dump_stack+0x0/0x28) from [ ] (panic+0xa4/0x1f8) [ ] (panic+0x0/0x1f8) from [ ] (mount_block_root+0x1c4/0x270) r3:00000001 r2:00000000 r1:c3843ea0 r0:c051a658 r7:c051a6c0 [ ] (mount_block_root+0x0/0x270) from [ ] (mount_root+0xec/0x114) [ ] (mount_root+0x0/0x114) from [ ] (prepare_namespace+0x160/0x1c0) r8:c05b3d40 r7:c0623dc0 r6:c05b3d68 r5:c05b3d68 r4:c0623e20 [ ] (prepare_namespace+0x0/0x1c0) from [ ] (kernel_init_freeable+0x1d8/0x220) r6:c0623dc0 r5:00000007 r4:c05d960c [ ] (kernel_init_freeable+0x0/0x220) from [ ] (kernel_init+0x10/0x158) [ ] (kernel_init+0x0/0x158) from [ ] (ret_from_fork+0x14/0x3c) r5:c046519c r4:00000000
nfs版本问题，我使用的linux系统版本是ubuntu18.04.5，。使用nfsstat命令查看nfs状态信息，确认host端nfs支持的版本。
ms@ubuntu:~$ nfsstat -v Server packet stats: packets udp tcp tcpconn 234 209 25 2 Server rpc stats: calls badcalls badfmt badauth badclnt 25 209 209 0 0 Server reply cache: hits misses nocache 0 0 25 Server io stats: read write 0 0 Server read ahead cache: size 0-10% 10-20% 20-30% 30-40% 40-50% 50-60% 60-70% 70-80% 80-90% 90-100% notfound 0 0 0 0 0 0 0 0 0 0 0 0 Server file handle cache: lookup anon ncachedir ncachenondir stale 0 0 0 0 0 Server nfs v3: null getattr setattr lookup access 2 8% 12 48% 0 0% 0 0% 2 8% readlink read write create mkdir 1 4% 0 0% 0 0% 0 0% 0 0% symlink mknod remove rmdir rename 0 0% 0 0% 0 0% 0 0% 0 0% link readdir readdirplus fsstat fsinfo 0 0% 0 0% 2 8% 0 0% 4 16% pathconf commit 2 8% 0 0% ms@ubuntu:~$三、解决方案。
修改内核配置，在bootargs中指定nfs版本，加上v3。
CONFIG_CMDLINE="noinitrd root=/dev/nfs nfsroot=198.168.1.18:/mnt/nfs/rootfs,v3,nolock rw ip=198.168.1.25:198.168.1.18:198.168.1.1:255.255.255.0::eth0:off init=/linuxrc console=ttyS0,115200n8 mem=64M"
成功启动，如下：
VFS: Mounted root (nfs filesystem) on device 0:11. devtmpfs: mounted Freeing unused kernel memory: 276K (c0594000 - c05d9000) BusyBox v1.22.1 (2016-02-03 14:11:04 CST) built-in shell (ash) Enter 'help' for a list of built-in commands. ~ #
