linux-kernel - Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

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Message-ID: <87v9obipk9.fsf@x220.int.ebiederm.org>
Date:   Wed, 12 Feb 2020 13:16:38 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Security Module <linux-security-module@...r.kernel.org>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman <ebiederm@...ssion.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?

The fundamental dcache thing we are playing with is:

	dentry = d_hash_and_lookup(proc_root, &name);
	if (dentry) {
		d_invalidate(dentry);
		dput(dentry);
	}

As Al pointed out upthread dput and d_invalidate can both sleep.

The dput can potentially go away if we use __d_lookup_rcu instead of
d_lookup.

The challenge is d_invalidate.

It has the fundamentally sleeping detach_mounts loop.  Even
shrink_dcache_parent has a cond_sched() in there to ensure it doesn't
live lock the system.

We could and arguabley should set DCACHE_CANT_MOUNT on the proc pid
dentries.  Which will prevent having to deal with mounts.

But I don't see an easy way of getting shrink_dcache_parent to run
without sleeping.  Ideas?


Eric