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

From: Suren Baghdasaryan <surenb@google.com>
To: Florian Weimer <fweimer@redhat.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>, Shakeel Butt <shakeelb@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, 7 Jul 2021 23:05:57 -0700	[thread overview]
Message-ID: <CAJuCfpFt55Dw1uW3S6_AincNfPaAtwdi6iXYVvFr7x3fvt4uzw@mail.gmail.com> (raw)
In-Reply-To: <87wnq1z7kl.fsf@oldenburg.str.redhat.com>

On Wed, Jul 7, 2021 at 10:41 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Suren Baghdasaryan:
>
> > On Wed, Jul 7, 2021 at 2:47 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * Suren Baghdasaryan:
> >>
> >> > The API is as follows,
> >> >
> >> >           int process_reap(int pidfd, unsigned int flags);
> >> >
> >> >         DESCRIPTION
> >> >           The process_reap() system call is used to free the memory of a
> >> >           dying process.
> >> >
> >> >           The pidfd selects the process referred to by the PID file
> >> >           descriptor.
> >> >           (See pidofd_open(2) for further information)
> >> >
> >> >           The flags argument is reserved for future use; currently, this
> >> >           argument must be specified as 0.
> >> >
> >> >         RETURN VALUE
> >> >           On success, process_reap() returns 0. On error, -1 is returned
> >> >           and errno is set to indicate the error.
> >>
> >> I think the manual page should mention what it means for a process to be
> >> “dying”, and how to move a process to this state.
> >
> > Thanks for the suggestion, Florian! Would replacing "dying process"
> > with "process which was sent a SIGKILL signal" be sufficient?
>
> That explains very clearly the requirement, but it raises the question
> why this isn't an si_code flag for rt_sigqueueinfo, reusing the existing
> system call.

I think you are suggesting to use sigqueue() to deliver the signal and
perform the reaping when a special value accompanies it. This would be
somewhat similar to my early suggestion to use a flag in
pidfd_send_signal() (see:
https://lore.kernel.org/patchwork/patch/1060407) to implement memory
reaping which has another advantage of operation on PIDFDs instead of
PIDs which can be recycled.
kill()/pidfd_send_signal()/sigqueue() are supposed to deliver the
signal and return without blocking. Changing that behavior was
considered unacceptable in these discussions. On the other hand using
some kthread to do the reaping asynchronously has its disadvantages:
userspace can't control the priority and cpu affinity of the thread
doing the reaping and this work is not charged towards the caller. In
the end a separate blocking syscall was deemed appropriate for this
operation. More details can be found in the links I posted in the
description of the patch.

>
> Thanks,
> Florian
>