linux-kernel - Re: [PATCH v1 05/29] mm/balloon_compaction: make PageOffline sticky until the page is freed

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Message-ID: <00d8e03d-c36f-47b6-919f-00a411ea6fd8@redhat.com>
Date: Tue, 1 Jul 2025 10:21:34 +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 05/29] mm/balloon_compaction: make PageOffline sticky
 until the page is freed

On 30.06.25 18:01, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:46PM +0200, David Hildenbrand wrote:
>> Let the page freeing code handle clearing the page type.
> 
> Why is this advantageous? We want to keep the page marked offline for longer?

Less code? ;)

I will add:

"Being able to identify balloon pages until actually freed is a 
requirement for upcoming movable_ops migration changes."

Note that the documentation is extended in patch #27 to mention that.

> 
>>
>> Acked-by: Zi Yan <ziy@...dia.com>
>> Acked-by: Harry Yoo <harry.yoo@...cle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> On assumption this UINT_MAX stuff is sane :)) I mean this is straightforward I
> guess:
 > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
>> ---
>>   include/linux/balloon_compaction.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
>> index b9f19da37b089..bfc6e50bd004b 100644
>> --- a/include/linux/balloon_compaction.h
>> +++ b/include/linux/balloon_compaction.h
>> @@ -140,7 +140,7 @@ static inline void balloon_page_finalize(struct page *page)
>>   		__ClearPageMovable(page);
>>   		set_page_private(page, 0);
>>   	}
>> -	__ClearPageOffline(page);
>> +	/* PageOffline is sticky until the page is freed to the buddy. */
> 
> OK so we are relying on this UINT_MAX thing in free_pages_prepare() to handle this.

Yes. Resetting the page_type -> _mapcount to the initial value -1.

-- 
Cheers,

David / dhildenb