From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs
 instances
Date: Wed, 12 Feb 2020 19:47:28 +0000
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>, LKML <linux-kernel@vger.kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>, Linux API <linux-api@vger.kernel.org>, Linux FS Devel <linux-fsdevel@vger.kernel.org>, Linux Security Module <linux-security-module@vger.kernel.org>, Akinobu Mita <akinobu.mita@gmail.com>, Alexey Dobriyan <adobriyan@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@kernel.org>, Daniel Micay <danielmicay@gmail.com>, Djalal Harouni <tixxdz@gmail.com>, "Dmitry V . Levin" <ldv@altlinux.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Ingo Molnar <mingo@kernel.org>, "J . Bruce Fields" <bfields@fieldses.org>, Jeff Layton <jlayton@poochiereds.net>, Jonathan Corbet <corbet@lwn.net>
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On Wed, Feb 12, 2020 at 10:45:06AM -0800, Linus Torvalds wrote:
> On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
> >
> > Fundamentally proc_flush_task is an optimization.  Just getting rid of
> > dentries earlier.  At least at one point it was an important
> > optimization because the old process dentries would just sit around
> > doing nothing for anyone.
> 
> I'm pretty sure it's still important. It's very easy to generate a
> _ton_ of dentries with /proc.
> 
> > I wonder if instead of invalidating specific dentries we could instead
> > fire wake up a shrinker and point it at one or more instances of proc.
> 
> It shouldn't be the dentries themselves that are a freeing problem.
> They're being RCU-free'd anyway because of lookup. It's the
> proc_mounts list that is the problem, isn't it?
> 
> So it's just fs_info that needs to be rcu-delayed because it contains
> that list. Or is there something else?

Large part of the headache is the possibility that some joker has
done something like mounting tmpfs on /proc/<pid>/map_files, or
binding /dev/null on top of /proc/<pid>/syscall, etc.

IOW, that d_invalidate() can very well have to grab namespace_sem.
And possibly do a full-blown fs shutdown of something NFS-mounted,
etc...