Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.03.2025 14:22, Alejandro Vallejo wrote:
> On Wed Mar 5, 2025 at 10:49 AM GMT, Jan Beulich wrote:
>> On 27.02.2025 15:36, Alejandro Vallejo wrote:
>>> On Wed Feb 26, 2025 at 2:05 PM GMT, Jan Beulich wrote:
>>>> On 24.02.2025 15:49, Alejandro Vallejo wrote:
>>>>> Open question to whoever reviews this...
>>>>>
>>>>> On Mon Feb 24, 2025 at 1:27 PM GMT, Alejandro Vallejo wrote:
>>>>>>      spin_lock(&heap_lock);
>>>>>> -    /* adjust domain outstanding pages; may not go negative */
>>>>>> -    dom_before = d->outstanding_pages;
>>>>>> -    dom_after = dom_before - pages;
>>>>>> -    BUG_ON(dom_before < 0);
>>>>>> -    dom_claimed = dom_after < 0 ? 0 : dom_after;
>>>>>> -    d->outstanding_pages = dom_claimed;
>>>>>> -    /* flag accounting bug if system outstanding_claims would go 
>>>>>> negative */
>>>>>> -    sys_before = outstanding_claims;
>>>>>> -    sys_after = sys_before - (dom_before - dom_claimed);
>>>>>> -    BUG_ON(sys_after < 0);
>>>>>> -    outstanding_claims = sys_after;
>>>>>> +    BUG_ON(outstanding_claims < d->outstanding_pages);
>>>>>> +    if ( pages > 0 && d->outstanding_pages < pages )
>>>>>> +    {
>>>>>> +        /* `pages` exceeds the domain's outstanding count. Zero it out. 
>>>>>> */
>>>>>> +        outstanding_claims -= d->outstanding_pages;
>>>>>> +        d->outstanding_pages = 0;
>>>>>
>>>>> While this matches the previous behaviour, do we _really_ want it? It's 
>>>>> weird,
>>>>> quirky, and it hard to extend to NUMA-aware claims (which is something in
>>>>> midway through).
>>>>>
>>>>> Wouldn't it make sense to fail the allocation (earlier) if the claim has 
>>>>> run
>>>>> out? Do we even expect this to ever happen this late in the allocation 
>>>>> call
>>>>> chain?
>>>>
>>>> This goes back to what a "claim" means. Even without any claim, a domain 
>>>> may
>>>> allocate memory. So a claim having run out doesn't imply allocation has to
>>>> fail.
>>>
>>> Hmmm... but that violates the purpose of the claim infra as far as I 
>>> understand
>>> it. If a domain may overallocate by (e.g) ballooning in memory it can 
>>> distort the
>>> ability of another domain to start up, even if it succeeded in its own 
>>> claim.
>>
>> Why would that be? As long as we hold back enough memory to cover the claim, 
>> it
>> shouldn't matter what kind of allocation we want to process. I'd say that a 
>> PV
>> guest starting ballooned ought to be able to deflate its balloon as far as
>> there was a claim established for it up front.
> 
> The fact a domain allocated something does not mean it had it claimed before. 
> A
> claim is a restriction to others' ability to allocate/claim, not to the domain
> that made the claim.

Yes and no. No in so far as the target's "ability to allocate" may or may not
be meant to extend beyond domain creation.

> e.g:
> 
>   0. host is idle. No domU.
>   1. dom1 is created with a claim to 10% of host memory and max_mem of 80% of
>      host meomory.
>   2. dom1 balloons in 70% of host memory.
>   3. dom1 ballons out 70% of host memory.
>   4. dom1 now holds a a claim to 80% of host memory.

Sure, this shouldn't be the result. Yet that may merely be an effect of claim
accounting being insufficient.

> It's all quite perverse. Fortunately, looking at adjacent claims-related code
> xl seems to default to making a claim prior to populating the physmap and
> cancelling the claim at the end of the meminit() hook so this is never a real
> problem.
> 
> This tells me that the logic intent is to force early failure of
> populate_physmap and nothing else. It's never active by the time ballooning or
> memory exchange matter at all.

Ah yes, this I find more convincing. (Oddly enough this is all x86-only code.)

> Xen ought to cancel the claim by itself though, toolstack should not NEED to 
> do
> it.

Fundamentally yes. Except that the toolstack can do it earlier than Xen could.

Jan