Re: [Xen-devel] [PATCH RFC] xen: prevent PVH Dom0 from having pages with more than one ref

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

On 13/06/14 13:05, Jan Beulich wrote:
>>>> Roger Pau MonnÃ<roger.pau@xxxxxxxxxx> 06/13/14 12:17 PM >>>
>> AFAICT (given my little knowledge about the domain builder), those pages
>> have been allocated by alloc_domheap_pages earlier in the code, which
>> makes them already have 1 ref, and then this loop adds another ref to
>> them, which makes no sense to me.
> 
> alloc_domheap_pages() (or really assign_pages()) takes one reference for
> setting PGC_allocated. This is the reference that gets dropped when freeing
> the page. The code you modify takes a second reference for the usage type
> of the page becoming writable, which is a result of it getting a writable page
> table entry made for it. That reference gets dropped when the respective
> page table entry gets cleared.
> 
>> The same happens for PV guests, but it doesn't seem to be a problem for
>> pure PV guests, and in fact removing this code for PV guests makes Xen
>> crash when the PV Dom0 boots. I would really appreciate if someone with
>> more knowledge on the domain builder code could comment on why this is
>> needed.
> 
> So other than Andrew I think the change is correct except for being only a
> partial fix - there's a second place further down where a similar piece of 
> code
> exists that would need fixing (conditionalizing) the same way.

Thanks, that makes sense, v2 on the way.

Roger.


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