Re: [PATCH 00/11] Introduce kernel_clone(), kill _do_fork()

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Aug 19, 2020 at 08:32:59AM -0500, Eric W. Biederman wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> 
> > On Wed, Aug 19, 2020 at 10:45:56AM +0200, Christian Brauner wrote:
> >> On Wed, Aug 19, 2020 at 09:43:40AM +0200, peterz@infradead.org wrote:
> >> > On Tue, Aug 18, 2020 at 06:44:47PM +0100, Matthew Wilcox wrote:
> >> > > On Tue, Aug 18, 2020 at 07:34:00PM +0200, Christian Brauner wrote:
> >> > > > The only remaining function callable outside of kernel/fork.c is
> >> > > > _do_fork(). It doesn't really follow the naming of kernel-internal
> >> > > > syscall helpers as Christoph righly pointed out. Switch all callers and
> >> > > > references to kernel_clone() and remove _do_fork() once and for all.
> >> > > 
> >> > > My only concern is around return type.  long, int, pid_t ... can we
> >> > > choose one and stick to it?  pid_t is probably the right return type
> >> > > within the kernel, despite the return type of clone3().  It'll save us
> >> > > some work if we ever go through the hassle of growing pid_t beyond 31-bit.
> >> > 
> >> > We have at least the futex ABI restricting PID space to 30 bits.
> >> 
> >> Ok, looking into kernel/futex.c I see 
> >> 
> >> pid_t pid = uval & FUTEX_TID_MASK;
> >> 
> >> which is probably what this referes to and /proc/sys/kernel/threads-max
> >> is restricted to FUTEX_TID_MASK.
> >> 
> >> Afaict, that doesn't block switching kernel_clone() to return pid_t. It
> >> can't create anything > FUTEX_TID_MASK anyway without yelling EAGAIN at
> >> userspace. But it means that _if_ we were to change the size of pid_t
> >> we'd likely need a new futex API. 
> >
> > Yes, there would be a lot of work to do to increase the size of pid_t.
> > I'd just like to not do anything to make that harder _now_.  Stick to
> > using pid_t within the kernel.
> 
> Just so people are aware.  If you look in include/linux/threads.h you
> can see that the maximum value of PID_MAX_LIMIT limits pids to 22 bits.
> 
> Further the design decisions of pids keeps us densly using pids.  So I
> expect it will be a while before we even come close to using 30 bits of
> pid space.

Also because it's simply annoying to have to type really large pid
numbers on the shell. Yes yes, that's a very privileged
developer-centric complaint but it matters when you have to do a quick
kill -9. Chromebook users obviously won't care about how large their
pids are for sure.

Tbf, related to discussions last year, systemd now actually raises the
default limit from ~33000 to 4194304. Which seems like an ok compromise.

Christian