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

[Zi Yan <ziy@nvidia.com>]

On 30 Sep 2025, at 11:20, Vlastimil Babka wrote:

> 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.

I thought so until I see the comment above page_private():
“page_private can be used on tail pages.”

Now I am not so confident about “it should be fine”.

>
>> 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.

Yes, I agree. But I think willy could give a more definitive answer here.

>
>>>
>>>> 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


Best Regards,
Yan, Zi