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, 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.

        Michael Young

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