Re: [Xen-devel] Xen BUG at page_alloc.c:1738 (Xen 4.5)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.05.15 at 20:06, <major@xxxxxxxx> wrote:
> I've been doing some testing of Xen 4.5 on Fedora 22 (due out within a week) 
> and I have an error that prevents the server from booting in the very early 
> boot process:
> 
>> (XEN) Xen call trace:
>> (XEN)    [<ffff82d08011d160>] free_domheap_pages+0x240/0x430
>> (XEN)    [<ffff82d08018c944>] mmio_ro_do_page_fault+0x114/0x160
>> (XEN)    [<ffff82d0801a4c10>] do_page_fault+0x1a0/0x4f0
>> (XEN)    [<ffff82d080239768>] handle_exception_saved+0x2e/0x6c
>> (XEN) 
>> (XEN) 
>> (XEN) ****************************************
>> (XEN) Panic on CPU 0:
>> (XEN) Xen BUG at page_alloc.c:1738
>> (XEN) ****************************************
> 
> The full output is over in a Github Gist[1].
> 
> I've tested this on some physical machines (Dell, HP, and SuperMicro 
> servers) as well as within a KVM virtual machine but I get the same boot 
> error each time.

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.

Jan


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