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

[Suren Baghdasaryan]

On Tue, Dec 8, 2020 at 3:40 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> 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.

Thanks for the comment Jann.
You are right. I think replacing !signal_group_exit(task->signal) with
task_will_free_mem(task) would address both your and Oleg's comments.
IIUC, task_will_free_mem() calls __task_will_free_mem() on the task
itself and on all processes sharing the mm_struct ensuring that they
are all dying.