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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
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