linux-kernel - Re: [PATCH v7 0/6] introduce PIDFD

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Message-ID: <CAKbZUD3w4_4MjrME-0mgRL01eFggb7et2BLa6012tzQX78KK9w@mail.gmail.com>
Date: Thu, 30 Jan 2025 23:10:53 +0000
From: Pedro Falcato <pedro.falcato@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian@...uner.io>, 
	Shuah Khan <shuah@...nel.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, linux-kselftest@...r.kernel.org, 
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Oliver Sang <oliver.sang@...el.com>, 
	John Hubbard <jhubbard@...dia.com>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutny <mkoutny@...e.com>, 
	Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v7 0/6] introduce PIDFD_SELF* sentinels

On Thu, Jan 30, 2025 at 10:53 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> 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@...cle.com> 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