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Thanks! I completely forgot about the malloc/in-place aspect.

Why the hash stack limit? I assume this precludes a fixed-size array.

I'm not sure when this sort happens, but early boot may not have a proper virtual memory implementation yet, so the kernel has to use a fixed stack size allocated by the boot loader.

Right, in this case specifically we're running off a limited bootstack allocated in the kernel's .bss (somewhere between 2 to 6 pages, generally); we won't finish initializing VM until shortly after this sort is done (in some of the SYSINITs that we're sorting).

Doesn't mergesort require malloc?

There are in-place array mergesorts. (Nobody uses them because they're horribly complicated, though.)

In this case however I'm putting all of the SYSINITs onto a linked list; mergesorting a linked list is easy.