Re: [PATCH v7 0/6] introduce PIDFD_SELF* sentinels

[Lorenzo Stoakes]

On Thu, Jan 30, 2025 at 03:32:36PM -0800, Andrew Morton wrote:
> 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)?

Ack, will expand if there's a respin, or Christian - perhaps you could fold
the above explanation into the cover letter?

Intent is for Christian to take this in his tree (if he so wishes) to be
clear!

Cheers