I am pretty surprised myself that I was able to consolidate
all "page table range" functions into a single type of iterator
(well, there are a couple of variations, but it's not too bad).

I thought at least the functions which allocate new page tables
would have to be seperate from those which don't.... it turns
out that if you slightly change the implementation of the
allocating functions, they start walking, quacking, etc. like
the other type.

This may also opens the way for things like:

#define for_each_pud(pgd, start, end, pud, pud_start, pud_end) \
if (pud = (pud_t *)pgd, pud_start = start, pud_end = end, 1)

for 2 and 3 level page tables (don't laugh if I've messed up the
above completely - you get the idea).

Then the last bit of the performance puzzle I think will just come
from inlining things. Now Andi doesn't want to do that (probably
rightly so)... what about compiling mm/ with -funit-at-a-time? It
doesn't do much harm to the stack, and it shaves off quite a few
KB...

Anyway, this iterator patch shaves off a bit itself. I haven't
looked at generated code, but I hope it is alright.

npiggin@didi:~/usr/src/linux-2.6$ size mm/memory.o.before
    text    data     bss     dec     hex filename
   11349       4     120   11473    2cd1 mm/memory.o
npiggin@didi:~/usr/src/linux-2.6$ size mm/memory.o.after
    text    data     bss     dec     hex filename
   11221       4     120   11345    2c51 mm/memory.o

Suggestions, help, etc. welcome.

Nick