linux-kernel - Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch added to -mm tree

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Date:	Thu, 10 Jul 2014 11:52:57 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Sasha Levin <sasha.levin@...cle.com>
cc:	Hugh Dickins <hughd@...gle.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
	davej@...hat.com, koct9i@...il.com, lczerner@...hat.com,
	stable@...r.kernel.org, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch
 added to -mm tree

On Thu, 10 Jul 2014, Sasha Levin wrote:
> On 07/10/2014 01:55 PM, Hugh Dickins wrote:
> >> And finally, (not) holding the i_mmap_mutex:
> > I don't understand what prompts you to show this particular task.
> > I imagine the dump shows lots of other tasks which are waiting to get an
> > i_mmap_mutex, and quite a lot of other tasks which are neither waiting
> > for nor holding an i_mmap_mutex.
> > 
> > Why are you showing this one in particular?  Because it looks like the
> > one you fingered yesterday?  But I didn't see a good reason to finger
> > that one either.
> 
> There are a few more tasks like this one, my criteria was tasks that lockdep
> claims were holding i_mmap_mutex, but are actually not.

You and Vlastimil enlightened me yesterday that lockdep shows tasks as
holding i_mmap_mutex when they are actually waiting to get i_mmap_mutex.
Hundreds of those in yesterday's log, hundreds of them in today's.

The full log you've sent (thanks) is for a different run from the one
you showed in today's mail.  No problem with that, except when I assert
that trinity-c190 in today's mail is just like trinity-c402 in yesterday's,
a task caught at one stage of exit_mmap in the stack dumps, then a later
stage of exit_mmap in the locks held dumps, I'm guessing rather than
confirming from the log.

There's nothing(?) interesting about those tasks, they're just tasks we
have been lucky to catch a moment before they reach the i_mmap_mutex
hang affecting the majority.

> 
> One new thing that I did notice is that since trinity spins a lot of new children
> to test out things like execve() which would kill said children, there tends to
> be a rather large amount of new tasks created and killed constantly.
> 
> So if you look at the bottom of the new log (attached), you'll see that there
> are quite a few "trinity-subchild" processes trying to die, unsuccessfully.

Lots of those in yesterday's log too: waiting to get i_mmap_mutex.

I'll pore over the new log.  It does help to know that its base kernel
is more stable: thanks so much.  But whether I can work out any more...

Hugh
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