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Date: Tue, 20 Nov 2012 12:14:39 -0200
From: Rafael Aquini <aquini@redhat.com>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>,
        Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH v12 4/7] mm: introduce compaction and migration for
 ballooned pages
Message-ID: <20121120141438.GA21672@x61.redhat.com>
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On Sun, Nov 18, 2012 at 09:59:47AM -0500, Sasha Levin wrote:
> On Sat, Nov 17, 2012 at 4:54 PM, Rafael Aquini <aquini@redhat.com> wrote:
> > On Sat, Nov 17, 2012 at 01:01:30PM -0500, Sasha Levin wrote:
> >>
> >> I'm getting the following while fuzzing using trinity inside a KVM tools guest,
> >> on latest -next:
> >>
> >> [ 1642.783728] BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> >> [ 1642.785083] IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> >>
> >> My guess is that we see those because of a race during the check in
> >> isolate_migratepages_range().
> >>
> >>
> >> Thanks,
> >> Sasha
> >
> > Sasha, could you share your .config and steps you did used with trinity? So I
> > can attempt to reproduce this issue you reported.
> 
> Basically try running trinity (with ./trinity -m --quiet --dangerous
> -l off) inside a disposable guest as root.
> 
> I manage to hit that every couple of hours.
> 
> Config attached.
> 

Howdy Sasha,

After several hours since last Sunday running trinity tests on a traditional
KVM-QEMU guest as well as running it on a lkvm guest (both running
next-20121115) I couldn't hit a single time the crash you've reported,
(un)fortunately.

Also, the .config you gave me, applied on top of next-20121115, haven't produced
the same bin you've running and hitting the mentioned bug, apparently.

Here's the RIP for your crash:
[ 1642.783728] BUG: unable to handle kernel NULL pointer dereference at
0000000000000194
[ 1642.785083] IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0


And here's the symbol address for the next-20121115 with your .config I've been
running tests on:
[raquini@x61 linux]$ nm -n vmlinux | grep isolate_migratepages_range 
ffffffff8122d890 T isolate_migratepages_range

Also, it seems quite clear I'm missing something from your tree, as applying the
RIP displacement (0x344) to my local isolate_migratepages_range sym addr leads
me to the _middle_ of a instruction opcode that does not dereference any
pointers at all.

So, if you're consistently reproducing the same crash, consider to share with us
a disassembled dump from the isolate_migratepages_range() you're running along
with the crash stack-dump, please.

Cheers!
-- Rafael

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