Message-ID: <20160503154656.GA27311@kroah.com>
Date:	Tue, 3 May 2016 08:46:56 -0700
From:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:	Steven Haigh <netwiz@....id.au>
Cc:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 4.4: INFO: rcu_sched self-detected stall on CPU

On Wed, May 04, 2016 at 01:11:46AM +1000, Steven Haigh wrote:
> On 03/05/16 06:54, gregkh@...uxfoundation.org wrote:
> > On Wed, Mar 30, 2016 at 05:04:28AM +1100, Steven Haigh wrote:
> >> Greg, please see below - this is probably more for you...
> >>
> >> On 03/29/2016 04:56 AM, Steven Haigh wrote:
> >>>
> >>> Interestingly enough, this just happened again - but on a different
> >>> virtual machine. I'm starting to wonder if this may have something to do
> >>> with the uptime of the machine - as the system that this seems to happen
> >>> to is always different.
> >>>
> >>> Destroying it and monitoring it again has so far come up blank.
> >>>
> >>> I've thrown the latest lot of kernel messages here:
> >>>      http://paste.fedoraproject.org/346802/59241532
> >>
> >> So I just did a bit of digging via the almighty Google.
> >>
> >> I started hunting for these lines, as they happen just before the stall:
> >> BUG: Bad rss-counter state mm:ffff88007b7db480 idx:2 val:-1
> >> BUG: Bad rss-counter state mm:ffff880079c638c0 idx:0 val:-1
> >> BUG: Bad rss-counter state mm:ffff880079c638c0 idx:2 val:-1
> >>
> >> I stumbled across this post on the lkml:
> >>     http://marc.info/?l=linux-kernel&m=145141546409607
> >>
> >> The patch attached seems to reference the following change in
> >> unmap_mapping_range in mm/memory.c:
> >>> -	struct zap_details details;
> >>> +	struct zap_details details = { };
> >>
> >> When I browse the GIT tree for 4.4.6:
> >> https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/mm/memory.c?id=refs/tags/v4.4.6
> >>
> >> I see at line 2411:
> >> struct zap_details details;
> >>
> >> Is this something that has been missed being merged into the 4.4 tree?
> >> I'll admit my kernel knowledge is not enough to understand what the code
> >> actually does - but the similarities here seem uncanny.
> > 
> > I'm sorry, I have no idea what you are asking me about here.  Did I miss
> > a patch that should be backported?  Did I backport something
> > incorrectly?
> 
> Hi Greg + all,
> 
> I did actually find the cause of my rss-counter problems - being the
> experimental PVH functionality in Xen. It caused a number of corruptions
> both on disk and in memory. Turning this off resolved the problem.
> 
> As for the 'fix' above. It seems there was talk that zap_details should
> be defined as { } to avoid a problem in newer versions of the kernel
> that was in linux-next.
> 
> The question that I cannot answer (and I leave this open to the more
> knowledgeable on the list than I) is if that fix should also be applied
> to other trees.
> 
> So the question as I see it:
> Is this an actual bug that we're just not seeing hit in other kernel
> versions - but the newer oom reaper code from linux-next uncovered it -
> or is the code as-is in the 4.4 tree considered correct?
> 
> It could well be that the experimental code in the Xen PVH was tickling
> something that triggered the same type of issue as per the original bug
> report leading to the patch quoted above.

I would recommend working with the xen developers, on their mailing
list, about this issue.  If you end up with a patch that needs to be
applied, please let me and stable@ know about it.

thanks,

greg k-h

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