Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx 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. At the same time I do agree that it makes sense to use a consistent type in the kernel to make it easier to read and update the code. Eric
