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

Matthew Wilcox
2020-08-19T11:19:16Z

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.