linux-kernel - Re: [PATCH v0] mm/page_alloc: Cleanup for __del_page_from_free

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Message-ID: <20251001043809.11019-1-jinji.z.zhong@gmail.com>
Date: Wed,  1 Oct 2025 04:38:08 +0000
From: jinji zhong <jinji.z.zhong@...il.com>
To: ziy@...dia.com
Cc: akpm@...ux-foundation.org,
	feng.han@...or.com,
	hannes@...xchg.org,
	jackmanb@...gle.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	liulu.liu@...or.com,
	mhocko@...e.com,
	surenb@...gle.com,
	vbabka@...e.cz,
	zhongjinji@...or.com
Subject: Re: [PATCH v0] mm/page_alloc: Cleanup for __del_page_from_free_list()

> 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,
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.

> 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@...or.com>
> >
> >Reviewed-by: Vlastimil Babka <vbabka@...e.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