Re: [Xen-devel] error in xen/arch/x86/mm.c:get_page during migration

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Feb 28, 2013, at 9:06 AM, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> On 28.02.13 at 12:56, Tim Deegan <tim@xxxxxxx> wrote:
>> At 09:52 -0500 on 25 Feb (1361785966), Andres Lagar-Cavilla wrote:
>>> There seems to be something else amiss though. Unless I am parsing
>>> this incorrectly, taf == PGT_writable | PGT_pae_xen_l2? And caf == PAT
>>> | PCD? Looks like a very unlikely combination
>> 
>> By my reading, 
>> 
>> taf = 0x7400000000000001 = typecount 1, PGT_writable_page | PGT_validated
>> caf = 0x0180000000000000 = refcount 0, PGC_state_free
> 
> Right.
D'oh :)

Sharing code never sets PGT_writable, only sets/clears PGT_shared. I tend to 
think the dom_cow rd belies a different problem. To use dom_cow, the domain has 
to have an explicit domctl that enables memory sharing.

Andres
> 
>> iow this is a free page but somehow has ended up with a typecount (which
>> explains why the get_page() failed).  And presumably this is one of the
>> various get_page[_and_type](page, dom_cow) calls in mem_sharing.c.
>> 
>> Since free_domheap_pages() has a BUG_ON(typecount != 0), it seems like
>> something's gone badly off the rails here. 
>> 
>> One place I can see that tinkers with typecount without holding a
>> ref is share_xen_page_with_guest(), which sets exactly this typecount,
>> but then calls page_set_owner(page, d).
>> 
>> There's some hairy code in __gnttab_map_grant_ref() too, but I _think_
>> it can't end up taking typecounts without refcounts.
>> 
>> __acquire_grant_for_copy() looks pretty hairy too, in particular this:
>>        (void)page_get_owner_and_reference(*page);
>> but presumably the matching put_page() would have crashed if that was
>> the problem.  Does anyone understand the grant code well enough to get
>> into that?
> 
> Problem is that the domain reported in the message is DOM_COW
> according to Olaf, yet he's not knowingly using page sharing. But
> that fact pretty much excluded the grant table or any other of the
> "usual" code paths for me (and I never looked at the sharing code,
> so hoped one of you two would).
> 
> Jan
> 


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