Re: [PATCH 1/1] mm: introduce process_reap system call

From: Shakeel Butt <shakeelb@google.com>
To: Suren Baghdasaryan <surenb@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>
Subject: Re: [PATCH 1/1] mm: introduce process_reap system call
Date: Wed, 30 Jun 2021 12:00:38 -0700	[thread overview]
Message-ID: <CALvZod6deRap_tE_dSPhQnpe7XNgQ6w9hZAEirRRB-bWBK+zBA@mail.gmail.com> (raw)
In-Reply-To: <CAJuCfpG4M=ZnqR9D9MPNB88nwWgQ9qA9Z9a6dymZ5abOxNucGg@mail.gmail.com>

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.