Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Wed, 12 Feb 2020 13:16:38 -0600
From: (Eric W. Biederman)
To: Linus Torvalds <>
Cc: LKML <>,  Kernel Hardening <>,  Linux API <>,  Linux FS Devel <>,  Linux Security Module <>,  Akinobu Mita <>,  Alexander Viro <>,  Alexey Dobriyan <>,  Andrew Morton <>,  Andy Lutomirski <>,  Daniel Micay <>,  Djalal Harouni <>,  "Dmitry V . Levin" <>,  Greg Kroah-Hartman <>,  Ingo Molnar <>,  "J . Bruce Fields" <>,  Jeff Layton <>,  Jonathan Corbet <>,  Kees Cook <>,  Oleg Nesterov <>,  Solar Designer <>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

Linus Torvalds <> writes:

> On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman <> 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) {

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

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?


Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.