Date: Fri, 17 Apr 2015 11:19:47 +0200
From: Michal Hocko <mhocko@suse.cz>
To: Andy Lutomirski <luto@amacapital.net>
Cc: David Herrmann <dh.herrmann@gmail.com>, Tom Gundersen <teg@jklm.no>,
        Havoc Pennington <hp@pobox.com>, Rik van Riel <riel@redhat.com>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Daniel Mack <daniel@zonque.org>, Djalal Harouni <tixxdz@opendz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1
Message-ID: <20150417091947.GA13951@dhcp22.suse.cz>
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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.
-- 
Michal Hocko
SUSE Labs
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