Date:	Mon, 11 May 2009 01:58:23 +0200
From:	Ulrich Lukas <stellplatz-nr.13a@...enparkplatz.de>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: whole system lock-up on low memory

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.


Attachement: one file, memory_overcommit.cc
(also here: http://datenparkplatz.de/DiesUndDas/memory_overcommit.cc)
---
#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");
}


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