linux-kernel - Re: process 'stuck' at exit.

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Message-ID: <20131210220251.GB5050@redhat.com>
Date:	Tue, 10 Dec 2013 17:02:51 -0500
From:	Dave Jones <davej@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhart@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: process 'stuck' at exit.

On Tue, Dec 10, 2013 at 01:57:49PM -0800, Linus Torvalds wrote:
 > On Tue, Dec 10, 2013 at 1:41 PM, Dave Jones <davej@...hat.com> wrote:
 > >
 > > http://codemonkey.org.uk/junk/trace
 > 
 > Hmm. Ok, so something is calling [__]get_user_pages_fast() and
 > put_page() in a loop, but the trace doesn't show what that "something"
 > is, because it is itself not ever called.
 > 
 > However, that pattern does seem to imply that the loop is in
 > get_futex_key(), because all the other loops I see seem to be calling
 > other things as well.
 > 
 > And the __get_user_pages_fast() call implies that it's the THP case
 > that triggers the "unlikely(PageTail(page))" case. And anyway,
 > otherwise we'd see lock_page()/unlock_page() too.
 > 
 > So it looks like __get_user_pages_fast() fails, and keeps failing.
 > Andrea, this is your code, any ideas? Commit a5b338f2b0b1f ("thp:
 > update futex compound knowledge") to be exact.

So, a reason that this might only be showing up now, is that in the last
week I added support to trinity to explicitly do huge page mmaps in the children,
whereas before it only ever did that with MAP_SHARED in the main pid, and then
every child inherited them on fork().

	Dave

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