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

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