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Date: Sun, 8 Aug 2021 09:13:30 -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 Fri, Aug 6, 2021 at 9:07 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> 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!
v8 is posted at https://lore.kernel.org/patchwork/patch/1473697/
Testing shows performance improvement from replacing mmget with mmgrab.
>
> > --
> > Michal Hocko
> > SUSE Labs
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