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

[Suren Baghdasaryan]

On Fri, Aug 6, 2021 at 9:07 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@xxxxxxxx> 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!

v8 is posted at https://lore.kernel.org/patchwork/patch/1473697/
Testing shows performance improvement from replacing mmget with mmgrab.

>
> > --
> > Michal Hocko
> > SUSE Labs