The paging design used on the x86-64 linux kernel port in 2.4.x provides:
o per process virtual address space limit of 512 Gigabytes
o top of userspace stack located at address 0x0000007fffffffff
o start of the kernel mapping = 0x0000010000000000
o global RAM per system 508*512GB=254 Terabytes
o no need of any common code change
o 512GB of vmalloc/ioremap space
Description:
x86-64 has a 4 level page structure, similar to ia32 PSE but with
some extensions. Each level consits of a 4K page with 512 64bit
entries. The levels are named in Linux PML4, PGD, PMD, PTE; AMD calls them
PML4E, PDPE, PDE, PTE respectively. For direct and kernel mapping
only 3 levels are used with the PMD pointing to 2MB pages.
Userspace is able to modify and it sees only the 3rd/2nd/1st level
pagetables (pgd_offset() implicitly walks the 1st slot of the 4th
level pagetable and it returns an entry into the 3rd level pagetable).
This is where the per-process 512 Gigabytes limit cames from.
The common code pgd is the PDPE, the pmd is the PDE, the
pte is the PTE. The PML4 remains invisible to the common
code.
Since the per-process limit is 512 Gigabytes (due to kernel common
code 3 level pagetable limitation), the higher virtual address mapped
into userspace is 0x7fffffffff and it makes sense to use it
as the top of the userspace stack to allow the stack to grow as
much as possible.
The kernel mapping and the direct memory mapping are split. Direct memory
mapping starts directly after userspace after a 512GB gap, while
kernel mapping is at the end of (negative) virtual address space to exploit
the kernel code model. There is no support for discontig memory, this
implies that kernel mapping/vmalloc/ioremap/module mapping are not
represented in their "real" mapping in mem_map, but only with their
direct mapped (but normally not used) alias.
Future:
During 2.5.x we can break the 512 Gigabytes per-process limit
possibly by removing from the common code any knowledge about the
architectural dependent physical layout of the virtual to physical
mapping.
Once the 512 Gigabytes limit will be removed the kernel stack will
be moved (most probably to virtual address 0x00007fffffffffff).
Nothing will break in userspace due that move, as nothing breaks
in IA32 compiling the kernel with CONFIG_2G.
Linus agreed on not breaking common code and to live with the 512 Gigabytes
per-process limitation for the 2.4.x timeframe and he has given me and Andi
some very useful hints... (thanks! :)
Thanks also to H. Peter Anvin for his interesting and useful suggestions on
the x86-64-discuss lists!
Current PML4 Layout:
Each CPU has an PML4 page that never changes.
Each slot is 512GB of virtual memory.
0 user space pgd or 40MB low mapping at bootup. Changed at context switch.
1 unmapped
2 __PAGE_OFFSET - start of direct mapping of physical memory
... direct mapping in further slots as needed.
509 some io mappings (others are in a memory hole below 4gb)
510 vmalloc and ioremap space
511 kernel code mapping, fixmaps and modules.
Other memory management related issues follows:
PAGE_SIZE:
If somebody is wondering why these days we still have a so small
4k pagesize (16 or 32 kbytes would be much better for performance
of course), the PAGE_SIZE have to remain 4k for 32bit apps to
provide 100% backwards compatible IA32 API (we can't allow silent
fs corruption or as best a loss of coherency with the page cache
by allocating MAP_SHARED areas in MAP_ANONYMOUS memory with a
do_mmap_fake). I think it could be possible to have a dynamic page
size between 32bit and 64bit apps but it would need extremely
intrusive changes in the common code as first for page cache and
we sure don't want to depend on them right now even if the
hardware would support that.
PAGETABLE SIZE:
In turn we can't afford to have pagetables larger than 4k because
we could not be able to allocate them due physical memory
fragmentation, and failing to allocate the kernel stack is a minor
issue compared to failing the allocation of a pagetable. If we
fail the allocation of a pagetable the only thing we can do is to
sched_yield polling the freelist (deadlock prone) or to segfault
the task (not even the sighandler would be sure to run).
KERNEL STACK:
1st stage:
The kernel stack will be at first allocated with an order 2 allocation
(16k) (the utilization of the stack for a 64bit platform really
isn't exactly the double of a 32bit platform because the local
variables may not be all 64bit wide, but not much less). This will
make things even worse than they are right now on IA32 with
respect of failing fork/clone due memory fragmentation.
2nd stage:
We'll benchmark if reserving one register as task_struct
pointer will improve performance of the kernel (instead of
recalculating the task_struct pointer starting from the stack
pointer each time). My guess is that recalculating will be faster
but it worth a try.
If reserving one register for the task_struct pointer
will be faster we can as well split task_struct and kernel
stack. task_struct can be a slab allocation or a
PAGE_SIZEd allocation, and the kernel stack can then be
allocated in a order 1 allocation. Really this is risky,
since 8k on a 64bit platform is going to be less than 7k
on a 32bit platform but we could try it out. This would
reduce the fragmentation problem of an order of magnitude
making it equal to the current IA32.
We must also consider the x86-64 seems to provide in hardware a
per-irq stack that could allow us to remove the irq handler
footprint from the regular per-process-stack, so it could allow
us to live with a smaller kernel stack compared to the other
linux architectures.
3rd stage:
Before going into production if we still have the order 2
allocation we can add a sysctl that allows the kernel stack to be
allocated with vmalloc during memory fragmentation. This have to
remain turned off during benchmarks :) but it should be ok in real
life.
Order of PAGE_CACHE_SIZE and other allocations:
On the long run we can increase the PAGE_CACHE_SIZE to be
an order 2 allocations and also the slab/buffercache etc.ec..
could be all done with order 2 allocations. To make the above
to work we should change lots of common code thus it can be done
only once the basic port will be in a production state. Having
a working PAGE_CACHE_SIZE would be a benefit also for
IA32 and other architectures of course.
vmalloc:
vmalloc should be outside the first 512GB to keep that space free
for the user space. It needs an own pgd to work on in common code.
It currently gets an own pgd in the 510th slot of the per CPU PML4.
PML4:
Each CPU as an own PML4 (=top level of the 4 level page hierarchy). On
context switch the first slot is rewritten to the pgd of the new process
and CR3 is flushed.
Modules:
Modules need to be in the same 4GB range as the core kernel. Otherwise
a GOT would be needed. Modules are currently at 0xffffffffa0000000
to 0xffffffffafffffff. This is inbetween the kernel text and the
vsyscall/fixmap mappings.
Vsyscalls:
Vsyscalls have a reserved space near the end of user space that is
acessible by user space. This address is part of the ABI and cannot be
changed. They have ffffffffff600000 to ffffffffffe00000 (but only
some small space at the beginning is allocated and known to user space
currently). See vsyscall.c for more details.
Fixmaps:
Fixed mappings set up at boot. Used to access IO APIC and some other hardware.
These are at the end of vsyscall space (ffffffffffe00000) downwards,
but are not accessible by user space of course.
Early mapping:
On a 120TB memory system bootmem could use upto 3.5GB
of memory for its bootmem bitmap. To avoid having to map 3.5GB by hand
for bootmem's purposes the full direct mapping is created before bootmem
is initialized. The direct mapping needs some memory for its page tables,
these are directly taken from the physical memory after the kernel. To
access these pages they need to be mapped, this is done by a temporary
mapping with a few spare static 2MB PMD entries.
Unsolved issues:
2MB pages for user space - may need to add a highmem zone for that again to
avoid fragmentation.
Andrea <andrea@suse.de> SuSE
Andi Kleen <ak@suse.de> SuSE
$Id: mm.txt,v 1.1.1.1 2004/08/02 10:49:27 ronald Exp $
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