> Cool. Sorry to be pestering about the 64-bit limits, but can we really
> use 2^64 bytes of memory on ia64/ppc64/x86-64 etc.? (AFAIK, 64-bit
> arches don't suffer from a small ZONE_LOWMEM.)
First of all, I'm not sure any of the 64-bit arches even fully support
64-bit physical addresses. If I remember correctly the first hammers
support 40 bits, with more to be added later. Power4 is in close to the
same boat, but I know they go up to 256GB today (I seem to recall
something about 44-bit being the limit, though).
Don't forget that highmem starts to be needed before the 4G boundary.
The kernel has only 1GB of virtual space (look for PAGE_OFFSET, which
defines it), which means that you start needing to pull all of the
highmem trickery before you get to the actual limits.
Nobody knows how far it will go. It's fairly safe to say that, at this
rate, Linux will keep up with whatever hardware anyone produces.
Unless, of course, someone gets even more perverse than PAE. :)
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