Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.02.2025 15:50, Alejandro Vallejo wrote:
> On Wed Feb 26, 2025 at 2:28 PM GMT, Roger Pau Monné wrote:
>> On Wed, Feb 26, 2025 at 03:08:33PM +0100, Jan Beulich wrote:
>>> On 26.02.2025 14:56, Roger Pau Monné wrote:
>>>> On Mon, Feb 24, 2025 at 01:27:24PM +0000, Alejandro Vallejo wrote:
>>>>> --- a/xen/common/page_alloc.c
>>>>> +++ b/xen/common/page_alloc.c
>>>>> @@ -490,13 +490,11 @@ static long outstanding_claims; /* total 
>>>>> outstanding claims by all domains */
>>>>>  
>>>>>  unsigned long domain_adjust_tot_pages(struct domain *d, long pages)
>>>>>  {
>>>>> -    long dom_before, dom_after, dom_claimed, sys_before, sys_after;
>>>>> -
>>>>>      ASSERT(rspin_is_locked(&d->page_alloc_lock));
>>>>>      d->tot_pages += pages;
>>>>>  
>>>>>      /*
>>>>> -     * can test d->claimed_pages race-free because it can only change
>>>>> +     * can test d->outstanding_pages race-free because it can only change
>>>>>       * if d->page_alloc_lock and heap_lock are both held, see also
>>>>>       * domain_set_outstanding_pages below
>>>>>       */
>>>>> @@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct domain 
>>>>> *d, long pages)
>>>>>          goto out;
>>>>
>>>> I think you can probably short-circuit the logic below if pages == 0?
>>>> (and avoid taking the heap_lock)
>>>
>>> Are there callers passing in 0?
>>
>> Not sure, but if there are no callers expected we might add an ASSERT
>> to that effect then.
>>
>>>>>      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;
>>>>> +    } else {
>>>>> +        outstanding_claims -= pages;
>>>>> +        d->outstanding_pages -= pages;
>>>>
>>>> I wonder if it's intentional for a pages < 0 value to modify
>>>> outstanding_claims and d->outstanding_pages, I think those values
>>>> should only be set from domain_set_outstanding_pages().
>>>> domain_adjust_tot_pages() should only decrease the value, but never
>>>> increase either outstanding_claims or d->outstanding_pages.
>>>>
>>>> At best the behavior is inconsistent, because once
>>>> d->outstanding_pages reaches 0 there will be no further modification
>>>> from domain_adjust_tot_pages().
>>>
>>> Right, at that point the claim has run out. While freeing pages with an
>>> active claim means that the claim gets bigger (which naturally needs
>>> reflecting in the global).
>>
>> domain_adjust_tot_pages() is not exclusively called when freeing
>> pages, see steal_page() for example.
>>
>> When called from steal_page() it's wrong to increase the claim, as
>> it assumes that the page removed from d->tot_pages is freed, but
>> that's not the case.  The domain might end up in a situation where
>> the claim is bigger than the available amount of memory.
>>
>> Thanks, Roger.
> 
> This is what I meant by my initial reply questioning the logic itself.
> 
> It's all very dubious with memory_exchange and makes very little sense on the
> tentative code I have for per-node claims.
> 
> I'd be quite happy to put an early exit before the spin_lock on pages <= 0.
> That also covers your initial comment and prevents claims from growing after a
> domain started running if it didn't happen to consume all of them.
> 
> Is anyone opposed?

We first need to reach common understanding what a claim is (or is not). See
the other reply just sent.

Jan