Re: [PATCH v0] mm/page_alloc: Cleanup for __del_page_from_free_list()

[Vlastimil Babka <vbabka@suse.cz>]

On 9/30/25 16:28, Zi Yan wrote:
> On 30 Sep 2025, at 9:55, Vlastimil Babka wrote:
> 
>> On 9/25/25 10:50, zhongjinji wrote:
>>> It is unnecessary to set page->private in __del_page_from_free_list().
>>>
>>> If the page is about to be allocated, page->private will be cleared by
>>> post_alloc_hook() before the page is handed out. If the page is expanded
>>> or merged, page->private will be reset by set_buddy_order, and no one
>>> will retrieve the page's buddy_order without the PageBuddy flag being set.
>>> If the page is isolated, it will also reset page->private when it
>>> succeeds.
>>
>> Seems correct.
> 
> This means high order free pages will have head[2N].private set to a non-zero
> value, where head[N*2].private is 1, head[N*(2^2)].private is 2, ...
> head[N*(2^M)].private is M and head[0].private is the actual free page order.

Hm right, tail pages...

> If such a high order free page is used as high order folio, it should be fine.

We don't reinterpret the private field in any of the first X tail pages for
folios? That would be bad too.

> But if user allocates a non-compound high order page and uses split_page()
> to get a list of order-0 pages from this high order page, some pages will
> have non zero private. I wonder if these users are prepared for that.
> 
> For example, kernel/events/ring_buffer.c does it. In its comment, it says
> “set its first page's private to this order; !PagePrivate(page) means it's
> just a normal page.”
> (see https://elixir.bootlin.com/linux/v6.17/source/kernel/events/ring_buffer.c#L634)
> 
> I wonder if non zero page->private would cause any issue there.
> 
> Maybe split_page() should set all page->private to 0.
> 
> Let me know if I get anything wrong.

Maybe we could postpone this optimization until struct pages are shrunk.

>>
>>> Since __del_page_from_free_list() is a hot path in the kernel, it would be
>>> better to remove the unnecessary set_page_private().
>>>
>>> Signed-off-by: zhongjinji <zhongjinji@honor.com>
>>
>> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
>>
>>> ---
>>>  mm/page_alloc.c | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index d1d037f97c5f..1999eb7e7c14 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -868,7 +868,6 @@ static inline void __del_page_from_free_list(struct page *page, struct zone *zon
>>>
>>>  	list_del(&page->buddy_list);
>>>  	__ClearPageBuddy(page);
>>> -	set_page_private(page, 0);
>>>  	zone->free_area[order].nr_free--;
>>>
>>>  	if (order >= pageblock_order && !is_migrate_isolate(migratetype))
> 
> 
> Best Regards,
> Yan, Zi