Re: [PATCH v2] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.03.2025 19:35, Alejandro Vallejo wrote:
> On Tue Mar 11, 2025 at 3:45 PM GMT, Jan Beulich wrote:
>> On 11.03.2025 16:42, Roger Pau Monné wrote:
>>> On Tue, Mar 11, 2025 at 02:53:04PM +0000, Alejandro Vallejo wrote:
>>>> On Tue Mar 11, 2025 at 12:46 PM GMT, Roger Pau Monné wrote:
>>>>> On Tue, Mar 04, 2025 at 11:10:00AM +0000, Alejandro Vallejo wrote:
>>>>>> The logic has too many levels of indirection and it's very hard to
>>>>>> understand it its current form. Split it between the corner case where
>>>>>> the adjustment is bigger than the current claim and the rest to avoid 5
>>>>>> auxiliary variables.
>>>>>>
>>>>>> Add a functional change to prevent negative adjustments from
>>>>>> re-increasing the claim. This has the nice side effect of avoiding
>>>>>> taking the heap lock here on every free.
>>>>>>
>>>>>> While at it, fix incorrect field name in nearby comment.
>>>>>>
>>>>>> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
>>>>>
>>>>> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>>
>>>> Thanks.
>>>>
>>>>> I think it would be nice to also ensure that once the domain is
>>>>> finished building (maybe when it's unpaused for the first
>>>>> time?) d->outstanding_pages is set to 0.  IMO the claim system was
>>>>> designed to avoid races during domain building, and shouldn't be used
>>>>> once the domain is already running.
>>>>>
>>>>> Thanks, Roger.
>>>>
>>>> As a matter of implementation that's already the case by toolstack being 
>>>> "nice"
>>>> and unconditionally clearing claims after populating the physmap.
>>>
>>> I see.  Another option would be to refuse the unpause a domain if it
>>> still has pending claims.  However I don't know how that will work out
>>> with all possible toolstacks.
>>>
>>>> However, I agree the hypervisor should do it on its own. I didn't find a
>>>> suitable place for it. 
>>>
>>> You could do it in arch_domain_creation_finished().
>>
>> Except that better wouldn't be arch-specific.
> 
> Why would it have to be arch-specific though? As far as the hypervisor is
> concerned, it doesn't seem to be.

Together with Roger's earlier clarification on his original remark, I fear
I don't understand the question: I asked that it not be arch-specific. And
Roger clarified that he also didn't mean it to be.

Jan