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

Eric W. Biederman
2020-08-19T13:36:58Z

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.

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