Re: physical memory limit of 64-bit linux

From: Ingo Molnar
Date: Tue Dec 16 2008 - 13:48:22 EST

* Rafael J. Wysocki <rjw@xxxxxxx> wrote:

> On Tuesday, 16 of December 2008, Martin Steigerwald wrote:
> >
> > Hi!
> >
> > What is the physical memory limit for 64-bit Linux? I read about 40
> > bit address bus for AMD Athlon X2 (1 TB) and 48 bit for Barcelona X4
> > (256 TB).
> >
> > Is 64-bit linux able to use that amount - provided that one would
> > manage to build it into a machine? Or does it have a lower limit?
> >
> > Looking into the Google crystal ball gives unclear pictures... I tend
> > to assume that Linux would handle it, but I am not sure.
> IIRC, the current maximal virtual memory space size of the kernel on
> x86_64 is 2^46.

Almost: the real current upstream kernel hard memory limit on x86-64 is 44
bits, i.e. 16 TB.

There's a couple of limits to consider here.

Firstly, there's the architectural limit imposed by the CPU - that is 48
bits, 256 TB. That is the full virtual memory range that x86-64 CPUs are
able to address: non-canonical addresses outside that range create an

I.e. valid addresses on x86-64 are in the range of:

[ 0xffff800000000000...0x00007fffffffffff ]

Which is minus 128 TB to plus 128 TB.

Traditionally (and because it's practical) that max range is split in two:
negative addresses to kernel-space-only addresses [the same on all tasks],
positive addresses to user-space addresses [unique to each process MM].

The kernel starts at minus 122 TB, far far down, to take maximum advantage
of the negative range:

#define __PAGE_OFFSET _AC(0xffff880000000000, UL)

(and we start with an 8 TB empty-mapped hole range. )

That is where all physical memory is mapped to, linearly. Then we have a
64 TB limit imposed on the maximum size of this linear kernel memory

#define MAXMEM _AC(0x00003fffffffffff, UL)

that is sized a bit optimistically - it ends at ffffc7ffffffffff, which
overlaps by 2TB into the vmalloc area, which starts at:

#define VMALLOC_START _AC(0xffffc20000000000, UL)

We want to set MAXMEM to VMALLOC_START-hole instead, where the hole is say
0x20000000000. (2 TB)

This problem is academic because there are no such systems in existence,
and because we have another limit on the size of physical memory:

# define MAX_PHYSMEM_BITS 44

So in reality MAXMEM should be limited to the max sparsemem-covered
physical memory range, via the patch below.

In terms of future extensibility:

phase 1) We could go to 45 bits (32 TB) via a twoliner patch,
should the need arise

phase 2) We can then go to 46 bits (64 TB) with small changes too - by
moving the vmalloc area up a notch and moving the followon
dynamic kernel mappings areas too.

phase 3) We could also go close to 47 bits: with various more invasive
movings of VMALLOC and rest upwards, and other considerations
such as the elimination of the generous start of 8 TB hole at
__PAGE_OFFSET - i.e. moving __PAGE_OFFSET straight down to
minus 128 TB. 120 TB would be doable.

phase 4) If the 48 bits limit is ever lifted on the CPU side, we can move
__PAGE_OFFSET down. This is actually less invasive than phase
3), because moving __PAGE_OFFSET is relatively easy. The far
more invasive change would be the necessary changes to the
virtual memory code: the current 4-level paging has a 256 TB
limit which comes from the 512*512*512*512*4K split of
pgd/pud/pmd/pte entries. Either PGDIR_SHIFT would have to be
increased, moving the root pgtable's size from 4K to 8K or more,
or another pgdir level would have to be introduced (which is
even more intrusive and much less likely to be implemented by hw