linux-kernel - Re: [PATCH v7 1/2] mm: introduce process

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Date:   Fri, 6 Aug 2021 09:07:28 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jan Engelhardt <jengelh@...i.de>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

On Thu, Aug 5, 2021 at 11:40 PM Michal Hocko <mhocko@...e.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