memory allocation mystery

memory allocation mystery

[Sasha Pachev]

Hello,

I have been investigating kernel behavior ( I am running 2.4.3) in out of 
memory conditions with swap completely disabled and discovered a rather 
interesting behavior. If you run the following code:

#include <stdio.h>
#include <stdlib.h>

#define LEAK_BLOCK (1024*1024)
#define MB (1024*1024)

int main()
{
  unsigned long total = 0;
  for (;;)
  {
    char* p, *p_end;
    if(!(p=malloc(LEAK_BLOCK)))
    {
      fprintf(stderr, "malloc() failed\n");
      exit(1);
    }
    p_end = p + LEAK_BLOCK;
    while(p < p_end)
      *p++ = 0;
    total += LEAK_BLOCK;
    printf("Allocated %d MB\n", total/MB);
  }
  
  return 0;   
}


the process eventually gets killed by the kernel, rather than getting an 
error from malloc() as you would logically expect

I have straced the process and see just a bunch of old_mmap() calls like this:

old_mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0)
= 0x46b6a000

( in addition to writes to stdout, of course). So it looks like old_mmap() 
never returns an error.

Can somebody explain this behavior? To me it looks like a bug...

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