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Date:   Wed, 14 Aug 2019 12:04:03 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Pavel Emelianov <xemul@virtuozzo.com>,
        Oleg Nesterov <oleg@redhat.com>
Cc:     Adrian Reber <areber@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Andrei Vagin (C)" <avagin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
Subject: Re: [PATCH v6 1/2] fork: extend clone3() to support setting a PID
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 <20190813143023.GC6971@redhat.com>
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On Wed, Aug 14, 2019 at 09:50:03AM +0000, Pavel Emelianov wrote:
> On 8/12/19 11:09 PM, Adrian Reber wrote:
> > The main motivation to add set_tid to clone3() is CRIU.
> > 
> > To restore a process with the same PID/TID CRIU currently uses
> > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > ns_last_pid and then (quickly) does a clone(). This works most of the
> > time, but it is racy. It is also slow as it requires multiple syscalls.
> > 
> > Extending clone3() to support set_tid makes it possible restore a
> > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > race free (as long as the desired PID/TID is available).
> > 
> > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > on clone3() with set_tid as they are currently in place for ns_last_pid.
> > 
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> 
> Acked-by: Pavel Emelyanov <xemul@openvz.org>

On Tue, Aug 13, 2019 at 04:30:24PM +0200, Oleg Nesterov wrote:
> On 08/12, Adrian Reber wrote:
> >
> > The main motivation to add set_tid to clone3() is CRIU.
> >
> > To restore a process with the same PID/TID CRIU currently uses
> > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > ns_last_pid and then (quickly) does a clone(). This works most of the
> > time, but it is racy. It is also slow as it requires multiple syscalls.
> >
> > Extending clone3() to support set_tid makes it possible restore a
> > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > race free (as long as the desired PID/TID is available).
> >
> > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > on clone3() with set_tid as they are currently in place for ns_last_pid.
> >
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> 
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> 

Added-to:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=pidfd

Merged-into:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=for-next

Thanks!
Christian