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Date:   Wed, 4 Aug 2021 15:50:24 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     mhocko@kernel.org, mhocko@suse.com, rientjes@google.com,
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        timmurray@google.com, linux-api@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
Subject: Re: [PATCH v6 1/2] mm: introduce process_mrelease system call
Message-Id: <20210804155024.e4e42e1b7b087937271fa7ce@linux-foundation.org>
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On Wed,  4 Aug 2021 11:50:03 -0700 Suren Baghdasaryan <surenb@google.com> 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?