Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

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?

> +    } else {
> +        outstanding_claims -= pages;
> +        d->outstanding_pages -= pages;
> +    }
>      spin_unlock(&heap_lock);
>  
>  out:

Cheers,
Alejandro