I made a google search about "anonymous inode" and it seems it's related to epoll ... but what actually is it?
At least in some contexts, an anonymous inode is an inode without an attached directory entry. The easiest way to create such an inode is as such:
int fd = open( "/tmp/file", O_CREAT | O_RDWR, 0666 );
unlink( "/tmp/file" );
// Note that the descriptor fd now points to an inode that has no filesystem entry; you
// can still write to it, fstat() it, etc. but you can't find it in the filesystem.