I have a number of machines which PXE boot using the images provided by CentOS here:

http://mirror.CentOS.org/CentOS/7/os/x86_64/images/pxeboot/

That gives the machines kernel 3.10.0-327.el7.x86_64. I would like to move to a newer kernel as I’m seeing a serious I/O problem which I
suspect is a kernel bug. I thought that would be as easy as grabbing the files from here:

http://mirror.CentOS.org/CentOS/7.2.1511/os/x86_64/images/pxeboot/

… but that’s not the case, those are the same exact files as the earlier ones. These machines always PXE boot, they have no OS installed to their local disk. Instead their root filesystem is on an NFS
server. So this is not PXE+kickstart or other provisioning system, just PXE then mount an NFS export for its root filesystem.

So, how would one go about creating a new vmlinuz and initrd.img the same way that the ones shown above were built, except with a newer kernel? Also, what is upgrade.img?