Re: [GIT PULL] kdbus for 4.1-rc1

[Andy Lutomirski]

On Fri, Apr 17, 2015 at 2:19 AM, Michal Hocko <mhocko@xxxxxxx> 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@xxxxxxxxx> wrote:
>> > Hi
>> >
>> > On Thu, Apr 16, 2015 at 4:34 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.

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.

In the kdbus model's favor, I think that allowing pages of data in the
receive queue to be swapped out is potentially quite nice, but I'm
less convinced about non-full pages in the receive queue.  There's a
resource management tradeoff here, and one nice thing about AF_UNIX is
that sends are genuinely non-blocking.

--Andy
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