On Thu, 30 Jan 2025 23:10:53 +0000 Pedro Falcato <pedro.falcato@xxxxxxxxx> wrote:
> On Thu, Jan 30, 2025 at 10:53 PM Lorenzo Stoakes
> <lorenzo.stoakes@xxxxxxxxxx> wrote:
> >
> > > The above code sequence doesn't seem at all onerous. I'm not
> > > understanding why it's worth altering the kernel to permit this little
> > > shortcut?
> >
> > In practice it adds quite a bit of overhead for something that whatever
> > mechanism is using the pidfd can avoid.
> >
> > It was specifically intended for a real case of utilising
> > process_madvise(), using the newly extended ability to batch _any_
> > madvise() operations for the current process, like:
> >
> > if (process_madvise(PIDFD_SELF, iovec, 10, MADV_GUARD_INSTALL, 0)) {
> > ... error handling ...
> > }
> >
> > vs.
> >
> > pid_t pid = getpid();
> > int pidfd = pidfd_open(pid, PIDFD_THREAD);
> >
> > if (pidfd < 0) {
> > ... error handling ...
> > }
> >
> > if (process_madvise(PIDFD_SELF, iovec, 10, MADV_GUARD_INSTALL, 0)) {
> > ... cleanup pidfd ...
> > ... error handling ...
> > }
> >
> > ...
> >
> > ... cleanup pidfd ...
> >
> > So in practice, it's actually a lot more ceremony and noise. Suren has been
> > working with this code in practice and found this to be useful.
>
> It's also nice to add that people on the libc/allocator side should
> also appreciate skipping pidfd_open's reliability concerns (mostly,
> that RLIMIT_NOFILE Should Not(tm) ever affect thread spawning or a
> malloc[1]). Besides the big syscall reduction and nice speedup, that
> is.
>
> [1] whether this is the already case is an exercise left to the
> reader, but at the very least we should not add onto existing problems
Thanks.
Could we please get all the above spelled out much more thoroughly in
the [0/n] description (aka Patch Series Sales Brochure)?
