Re: [PATCH v6 1/2] mm: introduce process_mrelease system call

From: Andrew Morton
Date: Wed Aug 04 2021 - 18:50:32 EST


On Wed, 4 Aug 2021 11:50:03 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:

> In modern systems it's not unusual to have a system component monitoring
> memory conditions of the system and tasked with keeping system memory
> pressure under control. One way to accomplish that is to kill
> non-essential processes to free up memory for more important ones.
> Examples of this are Facebook's OOM killer daemon called oomd and
> Android's low memory killer daemon called lmkd.
> For such system component it's important to be able to free memory
> quickly and efficiently. Unfortunately the time process takes to free
> up its memory after receiving a SIGKILL might vary based on the state
> of the process (uninterruptible sleep), size and OPP level of the core
> the process is running. A mechanism to free resources of the target
> process in a more predictable way would improve system's ability to
> control its memory pressure.
> Introduce process_mrelease system call that releases memory of a dying
> process from the context of the caller. This way the memory is freed in
> a more controllable way with CPU affinity and priority of the caller.
> The workload of freeing the memory will also be charged to the caller.
> The operation is allowed only on a dying process.
>
> After previous discussions [1, 2, 3] the decision was made [4] to introduce
> a dedicated system call to cover this use case.
>
> The API is as follows,
>
> int process_mrelease(int pidfd, unsigned int flags);
>
> DESCRIPTION
> The process_mrelease() system call is used to free the memory of
> an exiting process.
>
> The pidfd selects the process referred to by the PID file
> descriptor.
> (See pidofd_open(2) for further information)

I did s/pidofd_open/pidfd_open/

>
> The flags argument is reserved for future use; currently, this
> argument must be specified as 0.
>
> RETURN VALUE
> On success, process_mrelease() returns 0. On error, -1 is
> returned and errno is set to indicate the error.
>
> ERRORS
> EBADF pidfd is not a valid PID file descriptor.
>
> EAGAIN Failed to release part of the address space.
>
> EINTR The call was interrupted by a signal; see signal(7).
>
> EINVAL flags is not 0.
>
> EINVAL The memory of the task cannot be released because the
> process is not exiting, the address space is shared
> with another live process or there is a core dump in
> progress.
>
> ENOSYS This system call is not supported, for example, without
> MMU support built into Linux.
>
> ESRCH The target process does not exist (i.e., it has terminated
> and been waited on).
>
> ...
>
> mm/oom_kill.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 65 insertions(+)

The code is nice and simple.

Can we get a test suite into tools/testing/selftests?