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SubjectRe: For review: documentation of clone3() system call
Hello Christian,


>>>> These system calls create a new process, in a manner similar to
>>>> fork(2).
>>>> Unlike fork(2), these system calls allow the child process to
>>>> share parts of its execution context with the calling process,
>>> Hm, sharing part of the execution context is not the only thing that
>>> clone{3}() does.
>> True. That text has been in the page for 21 years. It probably needs
>> a new coat of paint...
>>> Maybe something like:
>>> Unlike fork(2), these system calls allow to create a child process with
>>> different properties than its parent. For example, these syscalls allow
>>> the child to share various parts of the execution context with the
>>> calling process such as [...]. They also allow placing the process in a
>>> new set of namespaces.
>>> Just a thought.
>> A good thought...
>> I changed the text to read:
>> Unlike fork(2), these system calls allow the child to be created
>> with various properties that differ from the parent. For example,
>> these system calls provide more precise control over what pieces
>> of execution context are shared between the calling process and
>> the child process. For example, using these system calls, the
>> caller can control whether or not the two processes share the vir‐
>> tual address space, the table of file descriptors, and the table
>> of signal handlers. These system system calls also allow the new
>> child process to placed in separate namespaces(7).
>> Okay?
> Nit: The second and thirs sentence both start with "For example".
> Otherwise sounds great.

You gotta love it when the nonnative speaker points out the style errors
of the native speaker :-).

I fixed this.


>>>> The stack argument specifies the location of the stack used by the
>>>> child process. Since the child and calling process may share mem‐
>>>> ory, it is not possible for the child process to execute in the
>>>> same stack as the calling process. The calling process must
>>>> therefore set up memory space for the child stack and pass a
>>>> pointer to this space to clone(). Stacks grow downward on all
>>> It might be a good idea to advise people to use mmap() to create a
>>> stack. The "canonical" way of doing this would usually be something like
>>> #define DEFAULT_STACK_SIZE (4 * 1024 * 1024) /* 8 MB usually on Linux */
>>> (Yes, the MAP_STACK is usally a noop but people should always include it
>>> in case some arch will have weird alignment requirement in which case
>>> this flag can be changed to actually do something...)
>> So, I'm getting a little bit of an education here, and maybe you are
>> going to further educate me. Long ago, I added the documentation of
>> MAP_STACK to mmap(2), but I never quite connected the dots.
>> However, you say MAP_STACK is *usually* a noop. As far as I can see,
>> in current kernels it is *always* a noop. And AFAICS, since it was first
>> added in 2.6.27 (2008), it has always been a noop.
>> I wonder if it will always be a noop.
>> If we go back and look at the commit:
>> [[
>> commit 2fdc86901d2ab30a12402b46238951d2a7891590
>> Author: Ingo Molnar <>
>> Date: Wed Aug 13 18:02:18 2008 +0200
>> x86: add MAP_STACK mmap flag
>> as per this discussion:
>> Pardo reported that 64-bit threaded apps, if their stacks exceed the
>> combined size of ~4GB, slow down drastically in pthread_create() - because
>> glibc uses MAP_32BIT to allocate the stacks. The use of MAP_32BIT is
>> a legacy hack - to speed up context switching on certain early model
>> 64-bit P4 CPUs.
>> So introduce a new flag to be used by glibc instead, to not constrain
>> 64-bit apps like this.
>> glibc can switch to this new flag straight away - it will be ignored
>> by the kernel. If those old CPUs ever matter to anyone, support for
>> it can be implemented.
>> ]]
>> And see also
>> So, my understanding from the above is that MAP_STACK was added to
>> allow a possible fix on some old architectures, should anyone decide it
>> was worth doing the work of implementing it. But so far, after 12 years,
>> no one did. It kind of looks like no one ever will (since those old
>> architectures become less and less relevant).
>> So, AFAICT, while it's not wrong to tell people to use mmap(MAP_STACKED),
>> it doesn't provide any benefit (and perhaps never will), and it is a
>> more clumsy than plain old malloc().
> Apart from MAP_STACK at some point (however unlikely) becoming relevant
> I also like that mmap() makes it explicit that the page needs to be
> readable (PROT_READ) and writeable (PROT_WRITE) and also every
> (relevant) libc I know uses the explicit mmap() in their pthread
> implementation. If you prefer the malloc() as an example that's
> obviously fine.

I think the benefits you point out are not so significant.
The default assumption is that a given piece of memory is
readable and writable.

Does musl use MAP_STACKED? AFAICT, it doesn't.

I suppose it's not that I prefer malloc(), but rather that (first I
was ignorant, and second that) I prefer simplicity. In fact I even
set up a branch to change various pieces of code I had to use
MAP_STACKED, and then had an "if I had thought about this for
30 minutes, I might have saved myself a couple of hours of work"
moment after I researched the history of MAP_STACKED a little.

I still don't know the answer for sure. I'm inclined to
leave things as they are until I hear a compelling
argument to change things.

>> Yeah, but I didn't get mentioned in the commit ;-)
> You provided your Ack after I had taken the patch into my tree and I've
> already changed it once for Arnd's ack so I didn't want to change it
> again.

Yes. But I *motivated* the commit :-). Just saying. I'm not
too concerned!


>> I changed the text here to read:
>> CLONE_PID (Linux 2.0 to 2.5.15)
>> If CLONE_PID is set, the child process is created with the
>> same process ID as the calling process. This is good for
>> hacking the system, but otherwise of not much use. From
>> Linux 2.3.21 onward, this flag could be specified only by
>> the system boot process (PID 0). The flag disappeared com‐
>> pletely from the kernel sources in Linux 2.5.16. Subse‐
>> quently, the kernel silently ignored this bit if it was
>> specified in the flags mask. Much later, the same bit was
>> recycled for use as the CLONE_PIDFD flag.
> Yip, sounds good.

Are you making fun of my accent? ;-)

Thanks again for your feedback, Christian.



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

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