As with file handles, the kernel allocates the inode structures dynamically, but can’t free them yet.
The value in inode-max denotes the maximum number of inode handlers. This value should be 3-4 times larger than the value in file-max, since stdin, stdout and network sockets also need an inode struct to handle them. When you regularly run out of inodes, you need to increase this value.
The file inode-nr contains the first two items from inode-state, so we’ll skip to that file…
Inode-state contains three actual numbers and four dummies. The actual numbers are, in order of appearance, nr_inodes, nr_free_inodes and preshrink.
Nr_inodes stands for the number of inodes the system has allocated, this can be slightly more than inode-max because Linux allocates them one pageful at a time.
Nr_free_inodes represents the number of free inodes (?) and preshrink is nonzero when the nr_inodes > inode-max and the system needs to prune the inode list instead of allocating more.
source