Re: [GIT PULL] kdbus for 4.1-rc1

Andy Lutomirski
2015-04-20T20:04:02Z

On Mon, Apr 20, 2015 at 5:43 AM, Michal Hocko <mhocko@suse.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@suse.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@gmail.com> wrote:
>> >> > Hi
>> >> >
>> >> > On Thu, Apr 16, 2015 at 4:34 PM, Andy Lutomirski <luto@amacapital.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