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Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages
- To: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 5 Mar 2025 11:52:51 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 05 Mar 2025 10:52:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.02.2025 15:59, Alejandro Vallejo wrote:
> On Wed Feb 26, 2025 at 2:02 PM GMT, Jan Beulich wrote:
>> On 24.02.2025 14:27, Alejandro Vallejo wrote:
>>> @@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct domain
>>> *d, long pages)
>>> goto out;
>>>
>>> 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 )
>>
>> The lhs isn't needed, is it? d->outstanding_pages is an unsigned quantity,
>> after all. Else dropping the earlier of the two BUG_ON() wouldn't be quite
>> right.
>
> d->outstanding pages is unsigned, but pages isn't.
>
> It was originally like that, but I then got concerned about 32bit machines,
> where you'd be comparing a signed and an unsigned integer of the same
> not-very-large width. That seems like dangerous terrains if the unsigned
> number
> grows large enough.
Oh, indeed - I didn't take ILP32 arches into account.
Jan
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