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

[jinji zhong <jinji.z.zhong@gmail.com>]

>On 1 Oct 2025, at 0:38, jinji zhong 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.
>>> If such a high order free page is used as high order folio, 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.
>>
>> Having non-empty page->private in tail pages of non-compound high-order
>> pages is not an issue, as pages from the pcp lists never guarantee their
>> initial state. If ensuring empty page->private for tail pages is required,
>
>Sure. But is it because all page allocation users return used pages with

Some users [2] do not reset the private back to 0. When the page is a tail
page, the non-zero private value will persist until the page is split.

>->private set back to 0? And can all page allocation users handle non-zero
>->private? Otherwise, it can cause subtle bugs.

Yes, you are right. Some users(like swapfile [1]) cannot handle non-zero private.

>> we should handle this in prep_new_page(), similar to the approach taken in
>> prep_compound_page().
>>
>>> 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)
>>
>> PagePrivate is a flag in page->flags that indicates page->private is
>> already in use. While PageBuddy serves a similar purpose, it additionally
>> signifies that the page is part of the buddy system.
>
>OK. You mean ->private will never be used if PagePrivate is not set
>in ring buffer code?

In the ring buffer code, it only uses the private field of the head page,
but I recently found that the swapfile [1] is assuming page->private is zero,
even if the page is a tail page, which seems a bit dangerous. Adding this
patch will make this situation worse.

link: https://elixir.bootlin.com/linux/v6.17/source/mm/swapfile.c#L3745 [1]
link: https://elixir.bootlin.com/linux/v6.17/source/mm/swapfile.c#L3887 [2]

>If you are confident about it is OK to make some pages’ ->private not being
>zero at allocation, I am not going to block the patch.
>
>>
>>> 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.
>>
>>>>
>>>>> 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