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

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Date:   Mon, 12 Jul 2021 14:51:54 +0200 (CEST)
From:   Jan Engelhardt <jengelh@...i.de>
To:     Suren Baghdasaryan <surenb@...gle.com>
cc:     Florian Weimer <fweimer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        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>,
        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 1/1] mm: introduce process_reap system call


On Thursday 2021-07-08 08:05, Suren Baghdasaryan wrote:
>>
>> 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.

The way I understood the request is that a userspace program (or perhaps two,
if so desired) should issue _two_ calls, one to deliver the signal,
one to perform the reap portion:

	uinfo.si_code = SI_QUEUE;
	sigqueue(pid, SIGKILL, &uinfo);
	uinfo.si_code = SI_REAP;
	sigqueue(pid, SIGKILL, &uinfo);