whole system lock-up on low memory

[Ulrich Lukas]

Hi,


vanilla linux 2.6.29.3, AMD64, tested on openSUSE 11.1 and Kubuntu 9.04.


I observe the following behaviour:

When any user application (non-kernel, non-root) consumes all the
available system memory, the system freezes completely instead of any
application being killed by the oom-killer.

The mouse pointer stalls, and there is a seemingly endless loop of
hard-disc access, even when no swap space on any harddisc is activated.

IMO, this is severe, because any application can practically crash the
system (e.g. in case of a memory-leak), causing data loss in case of
unsaved data.


To reproduce this, I've attached a small C++ utility
(compiles with g++ memory_overcommit.cc -o memory_overcommit.bin) which
allocates chunks of memory of user defined size.

On the Ubuntu system, the system freeze can be observed with swap
enabled on a cryptographic swap partition (dm-crypt; /etc/crypttab).
With openSUSE the lock-up also occurs with deactivated swap.
(swapoff -a).

Tested from a KDE terminal window as regular urser, the
steps to reproduce are:

* use cryptographic swap partition or disable swap
  (maybe also reproducible with normal swap; but apparently not on Suse)

* compile and invoke the attached code:
  ./memory_overcommit.bin

* enter a number (in MiB) of memory that is slightly smaller than the
available memory and press "enter" key once.

* enter a smaller number (minimum 1 MiB) and confirm again, do the same
again,..., successively approaching the limit of available memory with
smaller chunks.

* finally, when most of the memory/buffers/cache are used-up, the system
becomes unresponsive and constant, heavy harddisk-access commences.

Sometimes, killing the X-server or shutting down via hotkeys works after
several minutes of waiting, but this is not consistent.


I've reported this issue on the Ubuntu bug tracker before:
https://bugs.launchpad.net/ubuntu/+bug/283420

but as stated above, I also had the problem on another system.


I'd be glad if I could help if you need further information.
#include <cassert>
#include <cerrno>
#include <clocale>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <limits>

using std::exit;
using std::numeric_limits;
using std::printf;
using std::size_t;
using std::strlen;

void read_answer(char* buffer, size_t buffer_size)
{
	assert(buffer_size > 1);

	char const* ret = std::fgets(buffer, buffer_size, stdin);
	if(ret == 0)
	{
		printf("\nSorry, there was an error while reading the answer. This program will now terminate.\n");
		exit(EXIT_FAILURE);
	}
	else if(strlen(buffer) == buffer_size - 1 && buffer[buffer_size - 2] != '\n')
	{
		printf("Sorry, you answer is too long (possibly out of range). This program will now terminate.\n");
		exit(EXIT_FAILURE);
	}
}

size_t determine_size(char const* s)
{
	assert(strlen(s) > 0);

	char* endptr = 0;
	errno = 0;
	unsigned long v = std::strtoul(s, &endptr, 10);
	if(strlen(s) == 1 || endptr != s + strlen(s) - 1)
	{
		printf("\nSorry, your answer does not appear to be valid. This program will now terminate.\n");
		exit(EXIT_FAILURE);
	}
	else if((v == numeric_limits<unsigned long>::max() && errno == ERANGE) || v == 0
	        || numeric_limits<unsigned long>::max() / (1024 * 1024) < v)
	{
		printf("\nSorry, that value is out of range. This program will now terminate.\n");
		exit(EXIT_FAILURE);
	}
	v *= 1024 * 1024;
	
	assert(numeric_limits<size_t>::is_specialized);
	if(v > numeric_limits<size_t>::max() )
	{
		printf("\nSorry, that value is out of range. This program will now terminate.\n");
		exit(EXIT_FAILURE);
	}

	return v;
}

bool inquire_repeat(size_t& v)
{
	printf("\nShould a new allocation be made? You can enter:\n"
			"==> \"No\" to quit,\n"
			"==> Any Number of MiB to change the chunk size and continue, or\n"
			"==> Hit return to continue with last chunk size.\n\nYour answer: ");

	char answer[20];
	read_answer(answer, sizeof(answer) );
	if(std::strcmp(answer, "\n") == 0) {
		return true;
	}
	else if(std::strcmp(answer, "No\n") == 0) {
		return false;
	}
	
	v = determine_size(answer);
	return true;
}


int main() {
	#ifdef _POSIX_C_SOURCE
	std::setlocale(LC_ALL, ""); // Prepare thousands grouping in output.
	#endif
	
	printf("\n\nThis program allows to repeatedly allocate chunks of memory of user-specified size. "
	       "\nAfter each allocation the user can choose to repeat or to quit the program.\n\nFirst, please enter now "
	       "the amount of memory in MiB (1024 * 1024 bytes) \nto allocate in each round: ");
	char answer[20];
	read_answer(answer, sizeof(answer) );
	
	size_t alloc_size = determine_size(answer);

	size_t total = 0;
	do {
		printf("\n   >>> Starting to allocate chunk...\n");
		std::fflush(stdout);
		void* p = std::malloc(alloc_size);
		if(!p) {
			printf("\n   > The last memory allocation failed."
			       "\n   > This means the system reported the out-of-memory condition orderly."
			       "\nThis program will now terminate.\n");
			exit(0);
		}
		std::memset(p, 0, alloc_size);
		printf("   >>> A chunk was just allocated! <<<\n");
		if(numeric_limits<size_t>::max() - total >= alloc_size)
		{
			total += alloc_size;
			
			#ifdef _POSIX_C_SOURCE
			char const* fmt_string = "   The total number of bytes allocated is now %'zu.\n";
			#else
			char const* fmt_string = "   The total number of bytes allocated is now %zu.\n";
			#endif
			
			printf(fmt_string, total);
		}
		else {
			#ifdef _POSIX_C_SOURCE
			char const* fmt_string = "   More than %'zu bytes have been allocated in total by now.\n";
			#else
			char const* fmt_string = "   More than %zu bytes have been allocated in total by now.\n";
			#endif
			
			printf(fmt_string, total);
			total = numeric_limits<size_t>::max();
		}
	} while(inquire_repeat(alloc_size) );

	printf("\nQuit was requested. This program will now terminate.\n");
}