Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

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?

Cheers,
Alejandro