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If the page is expand= ed >>>> 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 o= rder. >> 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 wil= l >> 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 ->private set back to 0? And can all page allocation users handle non-zero ->private? Otherwise, it can cause subtle bugs. > we should handle this in prep_new_page(), similar to the approach taken i= n > prep_compound_page(). > >> For example, kernel/events/ring_buffer.c does it. In its comment, it say= s >> =E2=80=9Cset its first page's private to this order; !PagePrivate(page) = means it's >> just a normal page.=E2=80=9D >> (see https://elixir.bootlin.com/linux/v6.17/source/kernel/events/ring_bu= ffer.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? If you are confident about it is OK to make some pages=E2=80=99 ->private n= ot 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 woul= d be >>>> better to remove the unnecessary set_page_private(). >>>> >>>> Signed-off-by: zhongjinji >>> >>> Reviewed-by: Vlastimil Babka >>> >>>> --- >>>> 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(struc= t 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 >=3D pageblock_order && !is_migrate_isolate(migratetype)) > > >> Best Regards, >> Yan, Zi Best Regards, Yan, Zi