Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 05-08-21 10:08:58, Suren Baghdasaryan wrote:
> [...]
> > +     /*
> > +      * If the task is dying and in the process of releasing its memory
> > +      * then get its mm.
> > +      */
> > +     p = find_lock_task_mm(task);
> > +     if (!p) {
> > +             ret = -ESRCH;
> > +             goto put_pid;
> > +     }
> > +     if (task != p) {
> > +             get_task_struct(p);
> > +             put_task_struct(task);
> > +             task = p;
> > +     }
>
> Why do you need to take a reference to the p here? You are under
> task_lock so this will not go away and you only need p to get your mm.

True.

>
> > +
> > +     /* If the work has been done already, just exit with success */
> > +     if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
> > +             goto put_task;
>
> You want to release the task_lock

Missed it again :(

>
> > +
> > +     if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
>
> you want task_will_free_mem(p) and what is the point of the PF_KTHREAD
> check?

Yeah, looks like task_will_free_mem() covers that case already.

>
> > +             mm = task->mm;
> > +             mmget(mm);
>
> All you need is to make sure mm will not get released under your feet
> once task_lock is released so mmgrab is the right thing to do here. The
> address space can be torn down in parallel and that is OK and desirable.
>
> I think you really want something like this:
>
>         if (flags)
>                 return -EINVAL;
>
>         pid = pidfd_get_pid(fd, &f_flags);
>         if (IS_ERR(pid))
>                 return PTR_ERR(pid);
>         task = get_pid_task(pid, PIDTYPE_PID);
>         if (!task) {
>                 ret = -ESRCH;
>                 goto put_pid;
>         }
>
>         /*
>          * Make sure to chose a thread which still has a reference to mm
>          * during the group exit
>          */
>         p = find_lock_task_mm(task);
>         if (!p) {
>                 ret = -ESRCH;
>                 goto put_task;
>         }
>
>         mm = task->mm;
>         mmgrab(mm);
>         reap = true;
>         /* If the work has been done already, just exit with success */
>         if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
>                 reap = false;
>         } else if (!task_will_free_mem(p)) {
>                 reap = false;
>                 ret = -EINVAL;
>         }
>         task_unlock(p);
>
>         if (!reap)
>                 goto dropmm;;
>
>         /* Do the work*/
>
>
> dropmm:
>         mmdrop(mm);
> put_task:
>         put_task(task);
> put_pid:
>         put_pid(pid);
>
>         return ret;
>

This is indeed simpler to follow. I'll adopt your version. Thanks!

> --
> Michal Hocko
> SUSE Labs