Re: [PATCH v2 1/2] xen/balloon: set a mapping for ballooned out pages

[David Vrabel <david.vrabel@citrix.com>]

On 22/07/13 18:32, Ian Campbell wrote:
> On Mon, 2013-07-22 at 18:26 +0100, David Vrabel wrote:
>> On 22/07/13 18:22, Ian Campbell wrote:
>>> On Mon, 2013-07-22 at 17:51 +0100, David Vrabel wrote:
>>>> On 22/07/13 17:28, Stefano Stabellini wrote:
>>>>>
>>>>>  #ifdef CONFIG_HIGHMEM
>>>>>  #define inc_totalhigh_pages() (totalhigh_pages++)
>>>>> @@ -423,7 +426,8 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
>>>>>  		if (xen_pv_domain() && !PageHighMem(page)) {
>>>>>  			ret = HYPERVISOR_update_va_mapping(
>>>>>  				(unsigned long)__va(pfn << PAGE_SHIFT),
>>>>> -				__pte_ma(0), 0);
>>>>> +				pfn_pte(page_to_pfn(get_balloon_trade_page()),
>>>>> +					PAGE_KERNEL_RO), 0);
>>>>
>>>> Preemption needs to be disabled while using the trade page, see
>>>> suggestion below.
>>>
>>> Hopefully you mean just when setting up/manipulating it?
>>
>> Yes, sorry.
>>
>> get_...()
>> update_va_mapping()
>> put_...()
> 
> I can see why it would matter in the unmap_and_replace+fixup case (since
> you need them to happen "atomically") but why in this case? We don't
> actually care which of the trade pages gets used for this purpose, so
> even if we happen to get preempted and change CPU it doesn't really
> matter.

If a trade page from another CPU is used, it may concurrently have it's
MFN cleared by a unmap_and_replace call on the other CPU.

David