linux-kernel - Re: [PATCH v1 04/29] mm/page_alloc: let page freeing clear any set page type

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Message-ID: <d4d8b891-008d-4cbc-950f-2e44c4445904@redhat.com>
Date: Tue, 1 Jul 2025 10:17:13 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v1 04/29] mm/page_alloc: let page freeing clear any set
 page type

On 30.06.25 17:27, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:45PM +0200, David Hildenbrand wrote:
>> 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).
>>
>> In the future we might want to warn on some page types. Instead of
>> having an "allow list", let's rather wait until we know about once that
>> should go on such a "disallow list".
> 
> Is the idea here to get this to show up on folio dumps or?

As part of the netmem_desc series, there was a discussion about removing 
the mystical PP checks -- page_pool_page_is_pp() in page_alloc.c and 
replacing them by a proper page type check.

In that case, we would probably want to warn in case we get such a 
netmem page unexpectedly freed.

But, that page type does not exist yet in code, so the sanity check must 
be added once introduced.

> 
>>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>> Acked-by: Harry Yoo <harry.yoo@...cle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.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;
> 
> Feels like this could do with a comment!

/* Reset the page_type -> _mapcount to -1 */

-- 
Cheers,

David / dhildenb