Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

>>      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).

Jan