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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 4 Aug 2021 15:59:36 -0700
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Subject: Re: [PATCH v6 1/2] mm: introduce process_mrelease system call
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
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On Wed, Aug 4, 2021 at 11:50 AM 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)
>
>           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).
>
> [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

Next I wanna see cgroup.procs giving me pidfds.