Re: [PATCH 2/2] xen/mm: do not assign pages to a domain until they are scrubbed

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2026 17:54, Roger Pau Monné wrote:
> On Wed, Mar 25, 2026 at 05:26:01PM +0100, Roger Pau Monné wrote:
>> On Wed, Mar 25, 2026 at 03:56:05PM +0100, Jan Beulich wrote:
>>> On 25.03.2026 11:08, Roger Pau Monne wrote:
>>>>  * Disallow XENMEM_decrease_reservation until the domain has finished
>>>>    creation would fix the issue of pages being freed while pending scrub,
>>>>    but it's not clear there might be other usages that would be 
>>>> problematic,
>>>>    as get_page() on non-scrubbed pages would still return success.
>>>
>>> I agree this is of concern.
>>>
>>>> --- a/xen/common/memory.c
>>>> +++ b/xen/common/memory.c
>>>> @@ -388,6 +388,12 @@ static void populate_physmap(struct memop_args *a)
>>>>                              goto out;
>>>>                          }
>>>>                      }
>>>> +
>>>> +                    if ( assign_page(page, a->extent_order, d, memflags) )
>>>> +                    {
>>>> +                        free_domheap_pages(page, a->extent_order);
>>>
>>> The pages don't have an owner set yet, so that function will go straight
>>> to free_heap_pages(), needlessly passing "true" as last argument. Correct,
>>> but (for large pages, which the stashing is about) highly inefficient.
>>
>> My bad, I was sure I was using the same freeing function as
>> alloc_domheap_pages() on failure to assign, but I clearly wasn't.  I
>> will switch to using free_heap_pages().
> 
> Coming back to this, I can export free_heap_pages(), but then the call
> would also unconditionally have need_scrub == true, as the pages have
> been allocated without scrubbing.

But the assign_page() call is here to have the scrubbing done ahead of
it, so re-scrubbing after freeing shouldn't be necessary?

Jan