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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 16 Apr 2015 10:04:17 -0700
Message-ID: <CALCETrWPSMR9PPtAjJj=wQye-44c=7R-6VPu7i0kCtfjbMa_rg@mail.gmail.com>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1
To: David Herrmann <dh.herrmann@gmail.com>
Cc: 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>
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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.

--Andy
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