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

[Rafael Aquini <aquini@redhat.com>]

On Tue, Nov 20, 2012 at 08:18:04PM -0500, Sasha Levin wrote:
> On 11/20/2012 09:14 AM, Rafael Aquini wrote:
> > 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.
> 
> 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...
>

I still failing miserably on getting a reproducer, but I'm quite curious about
what kind of page is being left behind with this barbed wire at ->mapping.

This might be an overkill on messages, but could you run a test with the
following patch? This migth bring some light to this corner as well as it might
help me on figuring out a clever/cleaner way to perform that test.

Thanks a lot for your attention here.

---
diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compacti
index e339dd9..d47ac78 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -122,9 +122,10 @@ static inline bool balloon_page_movable(struct page *page)
         * this is not a page that uses ->mapping in a different way
         */
        if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
-           !page_mapped(page))
+           !page_mapped(page)) {
+               dump_page(page);
                return __is_movable_balloon_page(page);
-
+       }
        return false;
 }