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 J42buvI6 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 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. > > 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 f= or > folios? That would be bad too. I thought so until I see the comment above page_private(): =E2=80=9Cpage_private can be used on tail pages.=E2=80=9D Now I am not so confident about =E2=80=9Cit should be fine=E2=80=9D. > >> 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. >> >> 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) >> >> 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 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