Re: [PATCH 2/2] mm/madvise: add process_madvise MADV_DONTNEER support

[Jann Horn]

On Tue, Nov 24, 2020 at 6:50 AM 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.
> In such situation it is desirable to be able to free up the memory of the
> process being killed in a more controlled way.
> Enable MADV_DONTNEED to be used with process_madvise when applied to a
> dying process to reclaim its memory. This would allow userspace system
> components like oomd and lmkd to free memory of the target process in
> a more predictable way.
>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
[...]
> @@ -1239,6 +1256,23 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>                 goto release_task;
>         }
>
> +       if (madvise_destructive(behavior)) {
> +               /* Allow destructive madvise only on a dying processes */
> +               if (!signal_group_exit(task->signal)) {
> +                       ret = -EINVAL;
> +                       goto release_mm;
> +               }

Technically Linux allows processes to share mm_struct without being in
the same thread group, so I'm not sure whether this check is good
enough? AFAICS the normal OOM killer deals with this case by letting
__oom_kill_process() always kill all tasks that share the mm_struct.