Re: Syscall numbers for BProc

Christoph Hellwig
2003-04-10T06:17:34Z

On Sat, Apr 05, 2003 at 01:15:37PM -0700, hendriks@lanl.gov wrote:
> The reason it is the way it is because when I'm trying to avoid
> stomping on other syscalls, having a small foot print is a good thing.

Adding more syscalls isn't really a big deal - whether you add one or
a bunch of them in a diff doesn't really matter.

> Breaking out every call into a separate syscall number would also make
> it more difficult to add new features in the future.

Which is a good thing :) Having syscall multiplexers leads to very
messy APIs like the one you proposed :)

> Since our nodes are running *nothing* but the Bproc slave, you can't
> log in some other way to kill the slave and then reboot and you can't
> run shutdown -r or something like that becuase there are no init
> scripts.

We have a reboot notifier call chain in the kernel.

> > Should be read() on a special file.
>
> It started life like that but then I liked the idea of being able to
> do it from any node in the system. (remember no shared fs)

You have this no shared fs argument a few times - why don't you _add_
a shared virtual filesystem for kerne, information? This would clean
up many of the messier APIs.

> > I'm pretty sure this would better be a /proc/<pid>/image file you
> > can read from.
>
> I'm a little fuzzy on what you mean here. If you're suggesting that a
> process read from its own /proc/pid/image, then that's hard because
> the process is changing while you do it. In the 3rd party case (which
> vmadump doesn't support) it gets more tricky because you need to make
> sure the process is stopped and the CPU state stored while you're
> reading this.

Okay, you're right - this should be a syscall.

> VMADump doesn't depend on BProc at all. You will, however, need a
> system call for it the way it's written now :)

Yeah, conviencded. Care to submit a separated out vmadump aptch with
the syscalls for 2.5?

>
> > > 0x1030 - VMAD_LIB_CLEAR - clear the library list
> > > no arguments
> >
> > What library lists are all those calls about? Needs more explanation.
>
> If you look at the virtual memory space of a dynamically linked
> program, the percentage of space used by the program itself (i.e. not
> libraries) is often very small. In an effort to make process
> migration really cheap, we're willing to say that files X, Y and Z are
> available on the machine where we'll be restoring the process image.
> The candidates for remote caching are, obviously, large shared
> libraries.
>
> So, the dumper needs to know what it can expect to find on the remote
> system and what it can't. That's where the library list comes in. It
> probably should just be called the remote file list or something.
> It's a gross hack where we tell the kernel code what it doesn't need
> to dump. Anything that isn't dumped gets stored in the dump file as a
> reference to a file. (e.g. map X bytes of /lib/libc-2.3.2.so @ offset
> Y)
>
> And yeah, this might be cleaner as a writable special file but this
> was easy given the big syscall mux.

I don't think you really want a device for this. It's more an attribute
of the mapping, so a MAP_ALWAYS_LOCAL flag to mmap sounds like the right
thing.