Re: vmalloc limits in PPC kernels ?

On Thu, Feb 13, 2003 at 02:19:41PM -0600, vinai wrote:

I've been doing a little work trying to update an in-house custom driver
for x86.  We acquired fairly large chunks of data, and we can't afford
to lose any of it. To do this, we reserve a fairly large chunk of memory
in which the data can be stored, as we can't afford system latencies
when writing to disk.  We were able to this in 2.2 without any problems,
but in 2.4, a new "feature" was introduced to x86 that limited how much
memory one can "vmalloc".  I did a quick check, but didn't find anything
in the ppc asm directory.  Do we have such a limit ?

All architectures have a limit.  Its size is defined by
VMALLOC_END-VMALLOC_START and further limited by the size of each
preceding allocation (+ a PAGE_SIZE pad) before the allocation in
question.  Allocations also include ioremaps and on some architectures
consistent memory allocations absorb vmalloc space.

You can see the constraints in asm-ppc/pgtable.h.  On PPC, it is
possible to change the default parameters in order to expand the
amount of vmalloc space available.  This is accomplished via
config options under the advanced kernel options menu.  With
PAGE_OFFSET at 0x40000000 and highmem, I've run a system with
nearly 3GB of vmalloc space.

Regards,
--
Matt Porter
porter@cox.net
This is Linux Country. On a quiet night, you can hear Windows reboot.

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