linux-kernel - Re: [GIT PULL] kdbus for 4.1-rc1

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Date:	Mon, 20 Apr 2015 13:03:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	David Herrmann <dh.herrmann@...il.com>,
	Tom Gundersen <teg@...m.no>, Havoc Pennington <hp@...ox.com>,
	Rik van Riel <riel@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1

On Mon, Apr 20, 2015 at 5:43 AM, Michal Hocko <mhocko@...e.cz> wrote:
> On Fri 17-04-15 11:54:42, Andy Lutomirski wrote:
>> On Fri, Apr 17, 2015 at 2:19 AM, Michal Hocko <mhocko@...e.cz> wrote:
>> > On Thu 16-04-15 10:04:17, Andy Lutomirski wrote:
>> >> On Thu, Apr 16, 2015 at 8:01 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> >> > Hi
>> >> >
>> >> > On Thu, Apr 16, 2015 at 4:34 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> >> >> Whose memcg does the pool use?
>> >> >
>> >> > The pool-owner's (i.e., the receiver's).
>> >> >
>> >> >> If it's the receiver's, and if the
>> >> >> receiver can configure a memcg, then it seems that even a single
>> >> >> receiver could probably cause the sender to block for an unlimited
>> >> >> amount of time.
>> >> >
>> >> > How? Which of those calls can block? I don't see how that can happen.
>> >>
>> >> I admit I don't fully understand memcg, but vfs_iter_write is
>> >> presumably going to need to get write access to the target pool page,
>> >> and that, in turn, will need that page to exist in memory and to be
>> >> writable, which may need to page it in and/or allocate a page.  If
>> >> that uses the receiver's memcg (as it should), then the receiver can
>> >> make it block.  Even if it doesn't use the receiver's memcg, it can
>> >> trigger direct reclaim, I think.
>> >
>> > Yes, memcg direct reclaim might trigger but we are no longer waiting for
>> > the OOM victim from non page fault paths so the time is bounded. It
>> > still might a quite some time, though, depending on the amount of work
>> > done in the direct reclaim.
>>
>> Is that still true if OOM notifiers are involved?  I've lost track of
>> what changed there.
>
> memcg OOM is not triggered from get_user_pages. See 519e52473ebe (mm:
> memcg: enable memcg OOM killer only for user faults)
>
>> Any any event, I'm not entirely convinced that having a broadcast send
>> cause, say, PID 1 to block until an unbounded number of pages in a
>> potentially unbounded number of memcgs are reclaimed is a good idea.
>
> This deserves a clarification I guess. It is the memcg of the current
> task which gets charged during the page fault normally. So if PID1 tries
> to fault the memory in it will be its (most probably root) memcg which
> gets charged. If the memory was already charged to a different task's
> memcg and then it got swapped out, though, the PID1 would indeed wait
> for the reclaim in the target memcg to swap the page back in.
>
> In either case this sounds like a potential problem, because tasks
> could hide their memory charges from the limit or PID1 context could
> be blocked. But maybe I just misunderstood the and an uncharged memory
> cannot be used for the buffer.
>

Hmm.  One of the explicit design goals of kdbus is for sandboxing,
i.e. creating a restricted view ("endpoint") and letting sandboxed
things talk to non-sandboxed things outside through that restricted
view.

Given that, the ability for a broadcast receiver to cause a sender
(PID 1?) to allocate root-memcg pages seems like it could be a
problem.

--Andy
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