Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

["Roger Pau Monné" <roger.pau@citrix.com>]

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.