Re: [PATCH v7 08/14] xen/page_alloc: introduce preserved page flags macro

[Julien Grall <julien@xxxxxxx>]

Hi Carlo,

On 27/03/2024 11:10, Carlo Nonato wrote:
> Hi guys,
>
> > Question is: How would you justify such a change? IOW I'm not convinced
> > (yet) this wants doing there.
> You mean in this series?
>
> > Looking at the code, the flag is originally set in
> > alloc_domheap_pages(). So I guess it would make sense to do it in
> > free_domheap_pages().
> We don't hold the heap_lock there.
Regardless of the safety question (I will answer below), count_info is 
not meant to be protected by heap_lock. The lock is protecting the heap 
and ensure we are not corrupting them when there are concurrent call to 
free_heap_pages().
> Is it safe to change count_info without it?

count_info is meant to be accessed locklessly. The flag PGC_extra cannot 
be set/clear concurrently because you can't allocate a page that has not 
yet been freed.
So it would be fine to use clear_bit(..., ...);

Cheers,

--
Julien Grall