linux-kernel - Re: [PATCH] pid: remove unnecessary idr

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Message-ID: <aUF8Yif0CNxocq70@tycho.pizza>
Date: Tue, 16 Dec 2025 08:36:02 -0700
From: Tycho Andersen <tycho@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Oleg Nesterov <oleg@...hat.com>, Christian Brauner <brauner@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pid: remove unnecessary idr_preload()

On Tue, Dec 16, 2025 at 11:02:33AM +0100, Mateusz Guzik wrote:
> On Tue, Dec 16, 2025 at 10:44 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > On 12/15, Tycho Andersen wrote:
> > >
> > > From: "Tycho Andersen (AMD)" <tycho@...nel.org>
> > >
> > > As near as I can tell, nothing in the second pidmap_lock critical section
> > > actually allocates an id, so there is no need to do idr_preload().
> > > idr_replace() does not, nor does pidfs_add_pid().
> >
> > Yes. The extra idr_preload() was needed for pidfs_add_pid() after the commit
> > 9698d5a483654 ("pidfs: rework inode number allocation").
> >
> > But please see
> > [PATCH v3 2/2] pid: only take pidmap_lock once on alloc
> > https://lore.kernel.org/all/20251206131955.780557-3-mjguzik@gmail.com/
> >
> > This patch removes the unnecessary idr_preload() too.
> >
> 
> The patch can be trivially rebased on top of this one (actual rebase
> would be cumbersome; instead one can cp kernel/fork.c
> kernel/fork.c-tmp, apply this patch on top of stock file, mv
> kernel/fork.c-tmp kernel/fork.c and commit resulting diff).
> 
> However, unless someone is looking to put this into stable branches or
> similar, I think the dance can be skipped at no loss for future
> generations.

Yeah, agreed. I think let's just take yours. I'll test it now.

Tycho