Re: [PATCH v12 4/7] mm: introduce compaction and migration for ballooned pages

On 11/20/2012 09:14 AM, Rafael Aquini wrote:

On Sun, Nov 18, 2012 at 09:59:47AM -0500, Sasha Levin wrote:

quoted

On Sat, Nov 17, 2012 at 4:54 PM, Rafael Aquini [off-list ref] wrote:

quoted

On Sat, Nov 17, 2012 at 01:01:30PM -0500, Sasha Levin wrote:

quoted

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.

Odd... I can see it happening here every couple of hours.

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.

Yup, I carry another small fix to mpol (which is unrelated to this one).

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.

Sure!

The call chain is:

	isolate_migratepages_range
		balloon_page_movable
			__is_movable_balloon_page
				mapping_balloon

mapping_balloon() fails because it checks for mapping to be non-null (and it is -
it's usually a small value like 0x50), and then it dereferences that.

The relevant assembly is:

static inline int mapping_balloon(struct address_space *mapping)
{
        return mapping && test_bit(AS_BALLOON_MAP, &mapping->flags);
    17ab:       48 85 c0                test   %rax,%rax
    17ae:       0f 84 4c 02 00 00       je     1a00 <isolate_migratepages_range+0x590>
    17b4:       48 8b 80 40 01 00 00    mov    0x140(%rax),%rax
    17bb:       a9 00 00 00 20          test   $0x20000000,%eax
    17c0:       0f 84 3a 02 00 00       je     1a00 <isolate_migratepages_range+0x590>

It dies on 17b4.

Let me know if you need anything else from me, I can also add debug code into the
kernel if it would help you...


Thanks,
Sasha