powerpc64 malloc limit?

[Kostik Belousov]

On Wed, Nov 30, 2011 at 05:53:04PM +0100, Andreas Tobler wrote:
> On 30.11.11 17:22, Kostik Belousov wrote:
> >On Wed, Nov 30, 2011 at 06:24:41AM +0100, Andreas Tobler wrote:
> >>All,
> >>
> >>while working on gcc I found a very strange situation which renders my
> >>powerpc64 machine unusable.
> >>The test case below tries to allocate that much memory as 'wanted'. The
> >>same test case on amd64 returns w/o trying to allocate mem because the
> >>size is far to big.
> >>
> >>I couldn't find the reason so far, that's why I'm here.
> >>
> >>As Nathan pointed out the VM_MAXUSER_SIZE is the biggest on powerpc64:
> >>#define VM_MAXUSER_ADDRESS      (0x7ffffffffffff000UL)
> >>
> >>So, I'd expect a system to return an allocation error when a user tries
> >>to allocate too much memory and not really trying it and going to be
> >>unusable. Iow, I'd exepect the situation on powerpc64 as I see on amd64.
> >>
> >>Can anybody explain me the situation, why do I not have a working limit
> >>on powerpc64?
> >>
> >>The machine itself has 7GB RAM and 12GB swap. The amd64 where I compared
> >>has around 4GB/4GB RAM/swap.
> >>
> >>TIA,
> >>Andreas
> >>
> >>include<stdlib.h>
> >>#include<stdio.h>
> >>
> >>int main()
> >>{
> >>          void *p;
> >>
> >>          p = (void*) malloc (1152921504606846968ULL);
> >>          if (p != NULL)
> >>                  printf("p = %p\n", p);
> >>
> >>          printf("p = %p\n", p);
> >>          return (0);
> >>}
> >
> >First, you should provide details of what consistutes 'the unusable
> >machine situation' on powerpc.
> 
> I can not login anymore, everything is stuck except the core control 
> mechanisms for example the fan controller.
> 
> Top reports 'ugly' figures, below from a earlier try:
> 
> last pid:  6790;  load averages:  0.78,  0.84,  0.86    up 0+00:34:52
> 22:42:29 47 processes:  1 running, 46 sleeping
> CPU:  0.0% user,  0.0% nice, 15.4% system, 11.8% interrupt, 72.8% idle
> Mem: 5912M Active, 570M Inact, 280M Wired, 26M Cache, 104M Buf, 352K Free
> Swap: 12G Total, 9904M Used, 2383M Free, 80% Inuse, 178M Out
> 
>    PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU
> COMMAND
>   6768 andreast      1  52    01073741824G  6479M pfault  1   0:58
> 18.90% 31370.
> 
> And after my mem and swap are full I see swap_pager_getswapspace(16) failed.
> 
> In this state I can only power-cycle the machine.
> 
> >That said, on amd64 the user map is between 0 and 0x7fffffffffff, which
> >obviously less then the requested allocation size 0x100000000000000.
> >If you look at the kdump output on amd64, you will see that malloc()
> >tries to mmap() the area, fails and retries with obreak(). Default
> >virtual memory limit is unlimited, so my best quess is that on amd64
> >vm_map_findspace() returns immediately.
> >
> >On powerpc64, I see no reason why vm_map_entry cannot be allocated, but
> >please note that vm object and pages shall be only allocated on demand.
> >So I am curious how does your machine breaks and where.
> 
> I would expect that the 'system' does not allow me to allocate that much 
> of ram.

Does the issue with machine going into limbo reproducable with the code
you posted ?

Or, do you need to actually touch the pages in the allocated region ?

If the later (and I do expect that), then how many pages do you need
to touch before machine breaks ? Is it single access that causes the
havoc, or you need to touch the amount approximately equal to RAM+swap ?
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