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SysCall/syscall-6.md
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Limits on resources in Linux
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================================================================================
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Each process in the system uses certain amount of different resources like files, CPU time, memory and so on.
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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.
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We will start from userspace view and then we will look how it is implemented in the Linux kernel.
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There are three main fundamental [system calls](https://en.wikipedia.org/wiki/System_call) to manage resource limit for a process:
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* `getrlimit`
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* `setrlimit`
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* `prlimit`
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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](https://en.wikipedia.org/wiki/Process_identifier). Definitions of these functions looks:
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The `getrlimit` is:
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```C
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int getrlimit(int resource, struct rlimit *rlim);
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```
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The `setrlimit` is:
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```C
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int setrlimit(int resource, const struct rlimit *rlim);
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```
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And the definition of the `prlimit` is:
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```C
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int prlimit(pid_t pid, int resource, const struct rlimit *new_limit,
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struct rlimit *old_limit);
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```
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In the first two cases, functions takes two parameters:
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* `resource` - represents resource type (we will see available types later);
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* `rlim` - combination of `soft` and `hard` limits.
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There are two types of limits:
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* `soft`
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* `hard`
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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't exceed related `hard` limit never.
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Both these values are combined in the `rlimit` structure:
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```C
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struct rlimit {
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rlim_t rlim_cur;
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rlim_t rlim_max;
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};
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```
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The last one function looks a little bit complex and takes `4` arguments. Besides `resource` argument, it takes:
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* `pid` - specifies an ID of a process on which the `prlimit` should be executed;
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* `new_limit` - provides new limits values if it is not `NULL`;
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* `old_limit` - current `soft` and `hard` limits will be placed here if it is not `NULL`.
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Exactly `prlimit` function is used by [ulimit](https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html#index-ulimit) util. We can verify this with the help of [strace](https://linux.die.net/man/1/strace) util.
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For example:
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```
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~$ strace ulimit -s 2>&1 | grep rl
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prlimit64(0, RLIMIT_NPROC, NULL, {rlim_cur=63727, rlim_max=63727}) = 0
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prlimit64(0, RLIMIT_NOFILE, NULL, {rlim_cur=1024, rlim_max=4*1024}) = 0
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prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
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```
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Here we can see `prlimit64`, but not the `prlimit`. The fact is that we see underlying system call here instead of library call.
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Now let's look at list of available resources:
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| Resouce | Description
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|-------------------|------------------------------------------------------------------------------------------|
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| RLIMIT_CPU | CPU time limit given in seconds |
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| RLIMIT_FSIZE | the maximum size of files that a process may create |
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| RLIMIT_DATA | the maximum size of the process's data segment |
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| RLIMIT_STACK | the maximum size of the process stack in bytes |
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| RLIMIT_CORE | the maximum size of a [core](http://man7.org/linux/man-pages/man5/core.5.html) file. |
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| RLIMIT_RSS | the number of bytes that can be allocated for a process in RAM |
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| RLIMIT_NPROC | the maximum number of processes that can be created by a user |
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| RLIMIT_NOFILE | the maximum number of a file descriptor that can be opened by by a process |
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| RLIMIT_MEMLOCK | the maximum number of bytes of memory that may be locked into RAM by [mlock](http://man7.org/linux/man-pages/man2/mlock.2.html).|
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| RLIMIT_AS | the maximum size of virtual memory in bytes. |
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| RLIMIT_LOCKS | the maximum number [flock](https://linux.die.net/man/1/flock) and locking related [fcntl](http://man7.org/linux/man-pages/man2/fcntl.2.html) calls|
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| RLIMIT_SIGPENDING | maximum number of [signals](http://man7.org/linux/man-pages/man7/signal.7.html) that may be queued for a user of the calling process|
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| RLIMIT_MSGQUEUE | the number of bytes that can be allocated for [POSIX message queues](http://man7.org/linux/man-pages/man7/mq_overview.7.html) |
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| RLIMIT_NICE | the maximum [nice](https://linux.die.net/man/1/nice) value that can be set by a process |
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| RLIMIT_RTPRIO | maximum real-time priority value |
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| RLIMIT_RTTIME | maximum number of microseconds that a process may be scheduled under real-time scheduling policy without making blocking system call|
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If you're looking into source code of an open source projects, you will note that reading or updating of a resource limit is quite widely used operation and.
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For example: [systemd](https://github.com/systemd/systemd/blob/master/src/core/main.c)
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```C
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/* Don't limit the coredump size */
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(void) setrlimit(RLIMIT_CORE, &RLIMIT_MAKE_CONST(RLIM_INFINITY));
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```
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Or [haproxy](https://github.com/haproxy/haproxy/blob/master/src/haproxy.c):
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```C
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getrlimit(RLIMIT_NOFILE, &limit);
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if (limit.rlim_cur < global.maxsock) {
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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",
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argv[0], (int)limit.rlim_cur, global.maxconn, global.maxsock, global.maxsock);
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}
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```
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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.
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Limits on resource in the Linux kernel
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--------------------------------------------------------------------------------
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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:
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The `getrlimit`:
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```C
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SYSCALL_DEFINE2(getrlimit, unsigned int, resource, struct rlimit __user *, rlim)
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{
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struct rlimit value;
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int ret;
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ret = do_prlimit(current, resource, NULL, &value);
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if (!ret)
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ret = copy_to_user(rlim, &value, sizeof(*rlim)) ? -EFAULT : 0;
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return ret;
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}
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```
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and `setrlimit`:
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```C
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SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
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{
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struct rlimit new_rlim;
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if (copy_from_user(&new_rlim, rlim, sizeof(*rlim)))
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return -EFAULT;
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return do_prlimit(current, resource, &new_rlim, NULL);
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}
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```
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Implementations of these system calls are defined in the [kernel/sys.c](https://github.com/torvalds/linux/blob/master/kernel/sys.c) kernel source code file.
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First of all the `do_prlimit` function executes a check that the given resource is valid:
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```C
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if (resource >= RLIM_NLIMITS)
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return -EINVAL;
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```
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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:
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```C
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if (new_rlim) {
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if (new_rlim->rlim_cur > new_rlim->rlim_max)
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return -EINVAL;
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if (resource == RLIMIT_NOFILE &&
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new_rlim->rlim_max > sysctl_nr_open)
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return -EPERM;
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}
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```
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check that the given `soft` limit does not exceeds `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](https://en.wikipedia.org/wiki/Procfs):
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```
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~$ cat /proc/sys/fs/nr_open
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1048576
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```
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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:
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```C
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read_lock(&tasklist_lock);
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...
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...
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...
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read_unlock(&tasklist_lock);
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```
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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:
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```C
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rlim = tsk->signal->rlim + resource;
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```
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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:
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```C
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if (old_rlim)
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*old_rlim = *rlim;
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```
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That's all.
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Conclusion
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--------------------------------------------------------------------------------
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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](https://twitter.com/0xAX), drop me an [email](anotherworldofworld@gmail.com), or just create an [issue](https://github.com/0xAX/linux-internals/issues/new).
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**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](https://github.com/0xAX/linux-internals).**
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Links
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--------------------------------------------------------------------------------
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* [system calls](https://en.wikipedia.org/wiki/System_call)
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* [PID](https://en.wikipedia.org/wiki/Process_identifier)
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* [ulimit](https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html#index-ulimit)
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* [strace](https://linux.die.net/man/1/strace)
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* [POSIX message queues](http://man7.org/linux/man-pages/man7/mq_overview.7.html)
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