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

[Pedro Falcato]

On Thu, Jan 30, 2025 at 10:53 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> On Thu, Jan 30, 2025 at 02:37:54PM -0800, Andrew Morton wrote:
> > On Thu, 30 Jan 2025 20:40:25 +0000 Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote:
> >
> > > If you wish to utilise a pidfd interface to refer to the current process or
> > > thread it is rather cumbersome, requiring something like:
> > >
> > >     int pidfd = pidfd_open(getpid(), 0 or PIDFD_THREAD);
> > >
> > >     ...
> > >
> > >     close(pidfd);
> > >
> > > Or the equivalent call opening /proc/self. It is more convenient to use a
> > > sentinel value to indicate to an interface that accepts a pidfd that we
> > > simply wish to refer to the current process thread.
> > >
> >
> > 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

-- 
Pedro