Re: [RFC mm v6] mm: introduce a new page type for page pool in page type

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 17.11.25 17:02, Jesper Dangaard Brouer wrote:
> 
> On 17/11/2025 06.20, Byungchul Park wrote:
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 600d9e981c23..01dd14123065 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1041,7 +1041,6 @@ static inline bool page_expected_state(struct page *page,
>>    #ifdef CONFIG_MEMCG
>>    			page->memcg_data |
>>    #endif
>> -			page_pool_page_is_pp(page) |
>>    			(page->flags.f & check_flags)))
>>    		return false;
>>    
>> @@ -1068,8 +1067,6 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
>>    	if (unlikely(page->memcg_data))
>>    		bad_reason = "page still charged to cgroup";
>>    #endif
>> -	if (unlikely(page_pool_page_is_pp(page)))
>> -		bad_reason = "page_pool leak";
>>    	return bad_reason;
>>    }
> 
> This code have helped us catch leaks in the past.
> When this happens the result is that the page is marked as a bad page.
> 
>>    
>> @@ -1378,9 +1375,12 @@ __always_inline bool free_pages_prepare(struct page *page,
>>    		mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>>    		folio->mapping = NULL;
>>    	}
>> -	if (unlikely(page_has_type(page)))
>> +	if (unlikely(page_has_type(page))) {
>> +		/* networking expects to clear its page type before releasing */
>> +		WARN_ON_ONCE(PageNetpp(page));
>>    		/* Reset the page_type (which overlays _mapcount) */
>>    		page->page_type = UINT_MAX;
>> +	}
>>    
>>    	if (is_check_pages_enabled()) {
>>    		if (free_page_is_bad(page))
> 
> What happens to the page? ... when it gets marked with:
>     page->page_type = UINT_MAX
> 
> Will it get freed and allowed to be used by others?
> - if so it can result in other hard-to-catch bugs

Yes, just like most other use-after-free from any other subsystem in the 
kernel :)

The expectation is that such BUGs are found early during testing 
(triggering a WARN) such that they can be fixed early.

But we could also report a bad page here and just stop (return false).

-- 
Cheers

David