linux-insides/SysCall/linux-syscall-6.md

Limits on resources in Linux

Each process in the system uses certain amount of different resources like files, CPU time, memory and so on.

Such resources are not infinite and each process and we should have an instrument to manage it. Sometimes it is useful to know current limits for a certain resource or to change it's value. In this post we will consider such instruments that allow us to get information about limits for a process and increase or decrease such limits.

We will start from userspace view and then we will look how it is implemented in the Linux kernel.

There are three main fundamental system calls to manage resource limit for a process:

• getrlimit
• setrlimit
• prlimit

The first two allows a process to read and set limits on a system resource. The last one is extension for previous functions. The prlimit allows to set and read the resource limits of a process specified by PID. Definitions of these functions looks:

The getrlimit is:

int getrlimit(int resource, struct rlimit *rlim);

The setrlimit is:

int setrlimit(int resource, const struct rlimit *rlim);

And the definition of the prlimit is:

int prlimit(pid_t pid, int resource, const struct rlimit *new_limit,
            struct rlimit *old_limit);

In the first two cases, functions takes two parameters:

• resource - represents resource type (we will see available types later);
• rlim - combination of soft and hard limits.

There are two types of limits:

• soft
• hard

The first provides actual limit for a resource of a process. The second is a ceiling value of a soft limit and can be set only by superuser. So, soft limit can never exceed related hard limit.

Both these values are combined in the rlimit structure:

struct rlimit {
    rlim_t rlim_cur;
    rlim_t rlim_max;
};

The last one function looks a little bit complex and takes 4 arguments. Besides resource argument, it takes:

• pid - specifies an ID of a process on which the prlimit should be executed;
• new_limit - provides new limits values if it is not NULL;
• old_limit - current soft and hard limits will be placed here if it is not NULL.

Exactly prlimit function is used by ulimit util. We can verify this with the help of strace util.

For example:

~$ strace ulimit -s 2>&1 | grep rl

prlimit64(0, RLIMIT_NPROC, NULL, {rlim_cur=63727, rlim_max=63727}) = 0
prlimit64(0, RLIMIT_NOFILE, NULL, {rlim_cur=1024, rlim_max=4*1024}) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0

Here we can see prlimit64, but not the prlimit. The fact is that we see underlying system call here instead of library call.

Now let's look at list of available resources:

Resource	Description
RLIMIT_CPU	CPU time limit given in seconds
RLIMIT_FSIZE	the maximum size of files that a process may create
RLIMIT_DATA	the maximum size of the process's data segment
RLIMIT_STACK	the maximum size of the process stack in bytes
RLIMIT_CORE	the maximum size of a core file.
RLIMIT_RSS	the number of bytes that can be allocated for a process in RAM
RLIMIT_NPROC	the maximum number of processes that can be created by a user
RLIMIT_NOFILE	the maximum number of a file descriptor that can be opened by a process
RLIMIT_MEMLOCK	the maximum number of bytes of memory that may be locked into RAM by mlock.
RLIMIT_AS	the maximum size of virtual memory in bytes.
RLIMIT_LOCKS	the maximum number flock and locking related fcntl calls
RLIMIT_SIGPENDING	maximum number of signals that may be queued for a user of the calling process
RLIMIT_MSGQUEUE	the number of bytes that can be allocated for POSIX message queues
RLIMIT_NICE	the maximum nice value that can be set by a process
RLIMIT_RTPRIO	maximum real-time priority value
RLIMIT_RTTIME	maximum number of microseconds that a process may be scheduled under real-time scheduling policy without making blocking system call

If you're looking into source code of open source projects, you will note that reading or updating of a resource limit is quite widely used operation.

For example: systemd

/* Don't limit the coredump size */
(void) setrlimit(RLIMIT_CORE, &RLIMIT_MAKE_CONST(RLIM_INFINITY));

Or haproxy:

getrlimit(RLIMIT_NOFILE, &limit);
if (limit.rlim_cur < global.maxsock) {
	Warning("[%s.main()] FD limit (%d) too low for maxconn=%d/maxsock=%d. Please raise 'ulimit-n' to %d or more to avoid any trouble.\n",
		argv[0], (int)limit.rlim_cur, global.maxconn, global.maxsock, global.maxsock);
}

We've just saw a little bit about resources limits related stuff in the userspace, now let's look at the same system calls in the Linux kernel.

Limits on resource in the Linux kernel

Both implementation of getrlimit system call and setrlimit looks similar. Both they execute do_prlimit function that is core implementation of the prlimit system call and copy from/to given rlimit from/to userspace:

The getrlimit:

SYSCALL_DEFINE2(getrlimit, unsigned int, resource, struct rlimit __user *, rlim)
{
	struct rlimit value;
	int ret;

	ret = do_prlimit(current, resource, NULL, &value);
	if (!ret)
		ret = copy_to_user(rlim, &value, sizeof(*rlim)) ? -EFAULT : 0;

	return ret;
}

and setrlimit:

SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
{
	struct rlimit new_rlim;

	if (copy_from_user(&new_rlim, rlim, sizeof(*rlim)))
		return -EFAULT;
	return do_prlimit(current, resource, &new_rlim, NULL);
}

Implementations of these system calls are defined in the kernel/sys.c kernel source code file.

First of all the do_prlimit function executes a check that the given resource is valid:

if (resource >= RLIM_NLIMITS)
	return -EINVAL;

and in a failure case returns -EINVAL error. After this check will pass successfully and new limits was passed as non NULL value, two following checks:

if (new_rlim) {
	if (new_rlim->rlim_cur > new_rlim->rlim_max)
		return -EINVAL;
	if (resource == RLIMIT_NOFILE &&
			new_rlim->rlim_max > sysctl_nr_open)
		return -EPERM;
}

check that the given soft limit does not exceed hard limit and in a case when the given resource is the maximum number of a file descriptors that hard limit is not greater than sysctl_nr_open value. The value of the sysctl_nr_open can be found via procfs:

~$ cat /proc/sys/fs/nr_open 
1048576

After all of these checks we lock tasklist to be sure that signal handlers related things will not be destroyed while we updating limits for a given resource:

read_lock(&tasklist_lock);
...
...
...
read_unlock(&tasklist_lock);

We need to do this because prlimit system call allows us to update limits of another task by the given pid. As task list is locked, we take the rlimit instance that is responsible for the given resource limit of the given process:

rlim = tsk->signal->rlim + resource;

where the tsk->signal->rlim is just array of struct rlimit that represents certain resources. And if the new_rlim is not NULL we just update its value. If old_rlim is not NULL we fill it:

if (old_rlim)
    *old_rlim = *rlim;

That's all.

Conclusion

This is the end of the second part that describes implementation of the system calls in the Linux kernel. If you have questions or suggestions, ping me on Twitter 0xAX, drop me an email, or just create an issue.

Please note that English is not my first language and I am really sorry for any inconvenience. If you find any mistakes please send me PR to linux-insides.

Links

• system calls
• PID
• ulimit
• strace
• POSIX message queues