Ok, so what patch should be reverted?  I'm seeing other reports of
problems all around this same area, but I can't figure out exactly what
to do.

Are any of those reports public?  If so can you point me at them, I'm
curious if the symptoms match up.

I don't think we want to revert anything.  I think you should pull in
edc3b9129cec and at least the first three patches that Ben listed:

21cdb6b56843 x86/mm: Page align the '_end' symbol to avoid pfn conversion bugs
b61a76f8850d x86/efi: Map RAM into the identity page table for mixed mode
753b11ef8e92 x86/efi: Setup separate EFI page tables in kexec paths

the first patch takes a bit of massaging (mostly because some of it is
already touched by 02ff2769edbc, which keeps the changes from
edc3b9129cec, and so we can drop a good bit when applying).  The other
three apply cleanly.

I'm currently testing that and can send you the state of my tree in a bit.

 - R.