Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

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

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.

We might also break the invariant that total claims are strictly >=
total_avail_pages.

I'm somewhat puzzled at the "why" of having separate concepts for max_mem and
claims. I guess it simply grew the way it did. Reinstating sanity would
probably involve making max_mem effectively the claim, but that's a ton of
work I really would rather not do for now.

>
> 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.

A fair amount more, yes. I'm preparing a series on the side to address per-node
claims and it's far more invasive on page_alloc.c. This function was just
sufficiently impossible to read that I felt the urge to send it ahead of time
for my own mental health.

>
> Jan

Cheers,
Alejandro