Changing TASK_UNMAPPED_BASE to a low address

Thomas De Schampheleire patrickdepinguin+linuxppc at
Fri Oct 5 19:47:44 EST 2012


The current virtual address space of a 32-bit userspace process looks
like this on powerpc:
# cat /proc/28922/maps
00100000-00102000 r-xp 00000000 00:00 0          [vdso]
0fe70000-0ffd8000 r-xp 00000000 00:01 754        /lib/
0ffd8000-0ffe8000 ---p 00168000 00:01 754        /lib/
0ffe8000-0ffea000 r--p 00168000 00:01 754        /lib/
0ffea000-0ffed000 rwxp 0016a000 00:01 754        /lib/
0ffed000-0fff0000 rwxp 00000000 00:00 0
10000000-10001000 r-xp 00000000 00:12 73         /tmp/memory_alloc
10010000-10011000 rwxp 00000000 00:12 73         /tmp/memory_alloc
10011000-47a32000 rwxp 00000000 00:00 0          [heap]
48000000-4801d000 r-xp 00000000 00:01 793        /lib/
4801d000-4801f000 rw-p 00000000 00:00 0
4801f000-48021000 rw-p 00000000 00:00 0
4802d000-4802e000 r--p 0001d000 00:01 793        /lib/
4802e000-4802f000 rwxp 0001e000 00:01 793        /lib/
4802f000-bf6ee000 rw-p 00000000 00:00 0
bfa18000-bfa39000 rw-p 00000000 00:00 0          [stack]

The process text and data area are at 0x10000000 (fixed), the
mmap_base is at 0x48000000 (TASK_UNMAPPED_BASE = 3/8*0xc0000000), and
shared libraries are put directly above 0x10000000.

When your application starts allocating memory, the first blocks are
taken starting from mmap_base (0x48000000) until you reached the stack
area (0xbfa18000 in this example). Then, a 'heap' region is created
after the process text/data area (0x10011000 in this example) and the
mmap_base (0x48000000). When this is also filled, any further
allocation fails.
However, between 0x0 and the first shared library, there is still
plenty of space. Without any shared library, there would be approx.
256 MB free. Even with a few shared libraries, this typically is >200
MB, at least in the type of applications I'm envisioning.

Because of the 3GB/1GB userspace/kernelspace virtual address split,
one expects an application to be able to allocate close to 3GB = 3072
MB. Due to the 256MB 'unused' area mentioned, you can only allocate
something like 2800 MB. If your application needs more than that, and
you're bound to 32-bit processors, this is a problem.

I've been looking at two ways to use the extra memory:
1. do an anonymous mmap with an address hint in that range, e.g. at
address 0x00110000. This does return memory, and if you know how much
is free there, you can take all of it.

2. change the TASK_UNMAPPED_BASE setting in the kernel
(arch/powerpc/include/asm/processor.h) to a low address, e.g. 0x40000.
The advantage of this is that applications need no change.

The second approach seems to work fine. The memory map changes to:

# cat /proc/6954/maps
00040000-0005d000 r-xp 00000000 00:01 787        /lib/
0005d000-0005f000 rw-p 00000000 00:00 0
0005f000-00061000 rw-p 00000000 00:00 0
0006d000-0006e000 r--p 0001d000 00:01 787        /lib/
0006e000-0006f000 rwxp 0001e000 00:01 787        /lib/
00100000-00102000 r-xp 00000000 00:00 0          [vdso]
00102000-0c904000 rw-p 00000000 00:00 0
0ca00000-0ca21000 rw-p 00000000 00:00 0
0ca21000-0cb00000 ---p 00000000 00:00 0
0fe70000-0ffd8000 r-xp 00000000 00:01 750        /lib/
0ffd8000-0ffe8000 ---p 00168000 00:01 750        /lib/
0ffe8000-0ffea000 r--p 00168000 00:01 750        /lib/
0ffea000-0ffed000 rwxp 0016a000 00:01 750        /lib/
0ffed000-0fff0000 rwxp 00000000 00:00 0
10000000-10001000 r-xp 00000000 00:12 119
10010000-10011000 rwxp 00000000 00:12 119
10011000-cb82f000 rw-p 00000000 00:00 0          [heap]
bffaf000-bffd0000 rw-p 00000000 00:00 0          [stack]

I have not seen anything 'weird' about this change.
I found the following thread that also discusses such a change:
The thread talks about a few problems, but later concludes that these
have been fixed. Since the thread dates from 2004, I assume that
indeed this has been fixed.

Is there any reason why this change is not OK? I'm not necessarily
talking about mainlining the change. I first would like to know if it
is 'dangerous' or if there are other reasons not to do it.

This post is using linux- as base, but I looked at more recent
kernels and the settings seem the same.


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