[PATCH RFC 04/29] mm/page_alloc: allow for making page types sticky until freed

[David Hildenbrand]

On 18.06.25 20:08, Zi Yan wrote:
> On 18 Jun 2025, at 14:04, Zi Yan wrote:
> 
>> On 18 Jun 2025, at 13:39, David Hildenbrand wrote:
>>
>>> Let's allow for not clearing a page type before freeing a page to the
>>> buddy.
>>>
>>> We'll focus on having a type set on the first page of a larger
>>> allocation only.
>>>
>>> With this change, we can reliably identify typed folios even though
>>> they might be in the process of getting freed, which will come in handy
>>> in migration code (at least in the transition phase).
>>>
>>> Signed-off-by: David Hildenbrand <david at redhat.com>
>>> ---
>>>   mm/page_alloc.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 858bc17653af9..44e56d31cfeb1 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -1380,6 +1380,9 @@ __always_inline bool free_pages_prepare(struct page *page,
>>>   			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>>>   		page->mapping = NULL;
>>>   	}
>>> +	if (unlikely(page_has_type(page)))
>>> +		page->page_type = UINT_MAX;
>>> +
>>>   	if (is_check_pages_enabled()) {
>>>   		if (free_page_is_bad(page))
>>>   			bad++;
>>> -- 
>>> 2.49.0
>>
>> How does this preserve page type? Isn’t page->page_type = UINT_MAX clearing
>> page_type?
> 
> OK, next patch explains it. free_pages_prepare() clears page_type,
> so that caller does not need to.
> 
> I think the message is better to be
> 
> mm/page_alloc: clear page_type at page free time
> 
> page_type is no longer needed to be cleared before a page is freed, as
> page free code does that.
> 
> With this change, we can reliably identify typed folios even though
> they might be in the process of getting freed, which will come in handy
> in migration code (at least in the transition phase).


I'll change it to

     mm/page_alloc: let page freeing clear any set page type
     
     Currently, any user of page types must clear that type before freeing
     a page back to the buddy, otherwise we'll run into mapcount related
     sanity checks (because the page type currently overlays the page
     mapcount).
     
     Let's allow for not clearing the page type by page type users by letting
     the buddy handle it instead.
     
     We'll focus on having a page type set on the first page of a larger
     allocation only.
     
     With this change, we can reliably identify typed folios even though
     they might be in the process of getting freed, which will come in handy
     in migration code (at least in the transition phase).
     



-- 
Cheers,

David / dhildenb