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

[M A Young <m.a.young@xxxxxxxxxxxx>]

On Mon, 1 Jun 2015, M A Young wrote:

> On Mon, 1 Jun 2015, Jan Beulich wrote:
> 
> > >>> On 31.05.15 at 00:43, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > On 30/05/2015 23:07, M A Young wrote:
> > >> On Fri, 29 May 2015, Andrew Cooper wrote:
> > >>> FC22 is miscompiling the C to:
> > >>>
> > >>> struct page_info *page = mfn_to_page(mfn);
> > >>> struct domain *owner = page_get_owner_and_reference(page);
> > >>> if ( owner )
> > >>>     put_page(mfn_to_page(0));
> > >>>
> > >>> which is wrong, and why free_domheap_pages() does legitimately complain
> > >>> about the wonky refcount.
> > >> With a bit of experimentation I have found that compiling with the 
> > >> -fno-caller-saves flag gets this code segment back to the Fedora 21 
> > >> version, thus avoiding the bug.
> > > 
> > > After sending this email, I wondered whether the optimiser as assuming
> > > that %rdi was preserved.  Indeed, it turns out that the generated code
> > > for page_get_owner_and_reference leaves %rdi unmodified, and safe for
> > > reuse after return.
> > > 
> > > If the 'mov %r8,%rdi' were simply omitted, the code would work, as %rdi
> > > still contains the correct result of the original calculation.
> > 
> > And %r8 is known to be preserved too?
> > 
> > > Therefore, I suspect that the bug is in the -fcaller-saves optimisation
> > > code.
> > 
> > I suppose together with us allowing it to do such for global functions
> > by marking everything hidden (i.e. something possibly not seeing much
> > testing).
> > 
> > Questions now are:
> > 1) Was a bug against gcc opened already?
> > 2) What do we do about it? Working around the issue by setting
> > -fno-caller-saves seems awkward, as we'd likely have nothing but
> > the gcc version to tie this to. And considering distros carry their
> > own patch sets, the version alone may not even be enough. (I
> > didn't see any reports against our tip facing a similar issue despite
> > it being built with gcc 5 now too afaik.)
> 
> There is a Fedora bug on this
> https://bugzilla.redhat.com/show_bug.cgi?id=1219197
> which I updated and reassigned to gcc yesterday.

The Fedora gcc maintainer has now filed an upstream bug which is 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=66444

        Michael Young

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