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

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Message-ID: <38c64eec-0350-4e94-a115-76a569f4b934@redhat.com>
Date: Tue, 1 Jul 2025 12:02:39 +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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 Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v1 04/29] mm/page_alloc: let page freeing clear any set
 page type

On 01.07.25 10:37, Lorenzo Stoakes wrote:
> On Tue, Jul 01, 2025 at 10:34:33AM +0200, David Hildenbrand wrote:
>>>>>> Reviewed-by: Zi Yan <ziy@...dia.com>
>>>>>> Acked-by: Harry Yoo <harry.yoo@...cle.com>
>>>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Based on discussion below, I'm good with this now with the comment change, so
> feel free to add:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 */
>>>
>>> Hm this feels like saying 'the reason we set it to -1 is to set it to -1' :P
>>
>> Bingo! Guess why I didn't add a comment in the first place :P
>>
>>>
>>> I'd be fine with something simple like
>>>
>>> /* Set page_type to reset value */
>>
>> "Reset the page_type (which overlays _mapcount)"
>>
>> ?
> 
> Sounds good thanks, have an R-b above on the basis of this change.
> 
>>
>>>> But... Can't we just put a #define somewhere here to make life easy?
>> Like:
>>
>> Given that Willy will change all that soon, I'm not in favor of doing that
>> in this series.
> 
> Ah is he? I mean of course he is :))) this does seem like a prime target for the
> ongoing memdesc/foliofication efforts.

Right. According to the plans I know, the type will be stored as part of 
the memdesc pointer.

Clearing the type (where to clear, what to clear, when to clear) is 
probably the interesting bit in the future: probably it will be cleared 
as part of freeing any memdesc (thereby, invalidating the pointer).

-- 
Cheers,

David / dhildenb