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Subjectwhole 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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