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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 30 Jun 2021 12:06:39 -0700
Message-ID: <CAJuCfpE_d9w9vSbcvfDLTeVpEmTFWN65gSSyLntZYF51Bov+hg@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm: introduce process_reap system call
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Christoph Hellwig <hch@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jann Horn <jannh@google.com>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Wed, Jun 30, 2021 at 12:00 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Wed, Jun 30, 2021 at 11:44 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> [...]
> > > > +       /*
> > > > +        * If the task is dying and in the process of releasing its memory
> > > > +        * then get its mm.
> > > > +        */
> > > > +       task_lock(task);
> > > > +       if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
> > >
> > > task_will_free_mem() is fine here but I think in parallel we should
> > > optimize this function. At the moment it is traversing all the
> > > processes on the machine. It is very normal to have tens of thousands
> > > of processes on big machines, so it would be really costly when
> > > reaping a bunch of processes.
> >
> > Hmm. But I think we still need to make sure that the mm is not shared
> > with another non-dying process. IIUC that's the point of that
> > traversal. Am I mistaken?
>
> You are right. I am talking about efficiently finding all processes
> which are sharing mm (maybe linked into another list) instead of
> traversing all the processes on the system.

Oh, I see. I think that's a good idea but belongs to a separate patch
as an optimization for task_will_free_mem().
Thanks for reviewing and for good suggestions!