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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 12 Feb 2020 10:45:06 -0800
Message-ID: <CAHk-=wg52stFtUxMOxs3afkwDWmWn1JXC7RJ7dPsTrJbnxpZVg@mail.gmail.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
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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?

               Linus