Re: Xen BUG at page_alloc.c:1738 (Xen 4.5)

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 29/05/15 07:24, Jason Fritcher wrote:
> On Wed, 20 May 2015, Major Hayden wrote:
>
> >/ On 05/20/2015 05:41 AM, Jan Beulich wrote:/
> >/ > Considering that no-one else is seeing this - is this perhaps connected/
> >/ > to you building Xen with pre-release gcc 5.0.1? This is also because in/
> >/ > order for the above to indeed occur, mmio_ro_do_page_fault()'s/
> >/ > put_page() would need to drop the last reference of a page, yet/
> >/ > page_get_owner_and_reference() doesn't obtain a reference when/
> >/ > a page is unallocated (and hence unowned), i.e. normally a page/
> >/ > would have a refcount of at least 2 here. Hence this would be/
> >/ > possible only due to a race, but the exact same race to be observed/
> >/ > on different hardware _and_ under an emulator is extremely unlikely./
>
> You could try with the xen.gz file from
> https://copr-be.cloud.fedoraproject.org/results/myoung/xentest/fedora-21-x86_64/xen-4.5.1-0.rc1.fc21/xen-hypervisor-4.5.1-0.rc1.fc21.x86_64.rpm
> It is roughly the same version of xen but built against Fedora 21 and gcc
> 4.9.2. If that works then it probably is gcc 5.
> Greetings,
>
> I have run into pretty much the same issue as the original poster.
>
> I am running a recently updated Arch Linux system, with GCC 5.1.0,
> using UEFI and gummiboot to boot. I currently have a build of Xen
> 4.4.1, built with GCC 4.9.2 from before my last update, that is
> functioning correctly on this machine. But the builds of Xen 4.5.0,
> using GCC 5 and mingw64-binutils for the EFI binary, are all failing
> when Xen starts the Linux kernel, with the same error mentioned in the
> subject. Below is the boot log I captured via the serial port.
>
> http://pastebin.com/bBC78306
>
> Wondering if my specific toolchain was the issue, I downloaded the
> Fedora 22 version of xen-hypervisor and installed its EFI Xen binary
> over my compiled binary and received an identical error message, with
> slightly different addresses in the panic dump. The Fedora version was
> compiled with GCC 5.0.1. Below is the boot log I captured from that
> binary.
>
> http://pastebin.com/jvg1JazC
>
> After finding this thread, and specifically, the quoted message above,
> I downloaded that xen-hypervisor package and installed its EFI Xen
> binary. That binary boots successfully, as seen by the captured boot
> log below.
>
> http://pastebin.com/DKxwaU2U
>
> So, while I’m not familiar enough with Xen to begin to have an idea of
> what could possibly be wrong with Xen or GCC 5 to be causing this bug,
> I’d like to do what I can to track down the issue so I can get a
> working build of Xen 4.5. :)

Are you in a position to compile identical Xen 4.5 source with two
different versions of gcc?  (current staging-4.5 staging even has the
gcc5 build fix in)

If it is a gcc compiler bug, we would expect the version compiled with
gcc 4.9 to work fine, but the one compiled with 5 to fail in the
identified manor.

~Andrew

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