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.
Christian