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U2FsdGVkX19gEM5B4c9MNKfwM2sk3i7Yqvlfnza3xX/FdS0aZo11RSHYNy3eFD/PArBP47XUTZReXj9j9/w2fUANn9r8x5Df3GYWs/q6t3os4SOQ/KHdXW29dOI+A0FRNG2HMXCZnq7ywlOAyclJC1FAlyOoNpotBQzfXeWAdoZSchYgwfVeEU9PXMecyMq/izPn2WuHIqbtaXDEeuFBcXKcWTK6uqL9OVm4GlUJ0kQofiZJXAli9uhI1FLaFQHiuSVA850QO6N1FNPC8Rcx4DZGELHOmpSdZdH+kuxF0S95gOfrMOwXezpgrCmTtRL0R1bXhomtbO77DiBsoTpeuqcDH5AZKbOZpd6tgfcFOMGBoViINiCaMkOyIRSzbtYlGzbtYMn3ImVo5dno6PXNN2bk2EbWBwkT0tPPShxp57oMJtnV1d52Sd7zFoeqnshTasytTMK0VJTQTEwDU6hpHMRoZ8Rf/A2FtkvwJC+glJBsGn+0PIKdspj0CJ/ixZrCsLw06a1GP/d4HS2tyqsppOrx9QNtltCq+BZ+ygTJmlEq7xnMB9RLKPzeiu2gbTVHU4O2PY58Dx/uNrNrerRSPbmkufuRHnJcgNv70kTP9sUOAj04AYxclzVme8aKANSOZQyBUFvdPpG9l7z/nhH8XxL8mH6WSqhiyryx9WyaUkMzQBJd3ZgCq+SUlbmXuvB2XcnlxZHYy1NiOaUBlYUixahRSssQQ0NvBR0vBl5w3V8QdhvGs/vuESo0APo2f6ppOC902BTJPJTTsLt4iBPKrDrqLVwWbMbJjVoPxct7OGXfI/c+1bOshjH8BlnfEm9dSL7YHaEbxKftLu9LTCZWZ2L6mXXMybxco4dVBhv13Ns+X7R5UUv73kUqQO6YKsPGqhNv5m1s9ZkfiJGs6BfPZJ+GHT/lFN0Sw4CHl7Cb7wsgCkq7yFa47mXTgHrGm5wvJ8RUzP70by1puZyQI4s OjlJNtYu 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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 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 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