mmap() failed: Cannot allocate memory - how do I find out who's complaining?

Question

mmap() failed is popping up in my .xsession-errors in bursts. But there's no indication about what application it is failing with. How do I look into this after it has happened? I usually discover the failures only when I occasionally switch to my "tail -f .logs/*" workspace.

Answer 1

You can use SystemTap for this sort of thing.

• First, set up your system. You need SystemTap, headers corresponding to your running kernel, and debug symbols for the same. On Debian, getting it working is something like:

  sudo apt-get install systemtap
  sudo apt-get install linux-headers-$(uname -r)
  sudo apt-get install linux-image-$(uname -r)-dbg
  

  It can be a bit tricky to get it to work, because the kernel headers and symbols need to match your running kernel exactly. Hopefully it's straightforward on Gentoo.

• Then, save this file as mmap.stp:

  #! /usr/bin/env stap
  # -*- systemtap -*-
  
  global last_args
  
  probe syscall.mmap2 {
          last_args[tid()] = argstr;
  }
  
  probe syscall.mmap2.return {
          errno = errno_p(returnval())
          if (errno != 0) {
                  printf("%s[%d] mmap2(%s) failed: %d (%s)\n",
                         execname(), pid(), last_args[tid()],
                         errno, errno_str(errno));
          }
  }
  
  probe begin {
          printf("begin\n");
  }
  
  probe end {
          printf("end\n");
  }
  

• Run it with

  sudo stap mmap.stp
  

After it compiles and prints begin, It will watch every mmap() system call and save its arguments. If an error occurs, you'll get output that includes the program name, process ID, arguments, and error code. For example, running this nonsensical program:

int main() {
        mmap(1,-1,123,0,1,0);
}

gives the SystemTap output:

test[9601] mmap2(0x1, 4294967295, PROT_READ|PROT_WRITE|PROT_SEM|0x70, 0x0, 1, 0) failed: 22 (EINVAL)