Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

NUMA-aware claims require more than an adjustment just here, I expect. Tracking
of claims (certainly the global, maybe also the per-domain value) would likely
need to become per-node, for example.

Jan