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Tue, 02 Sep 2025 07:34:04 -0700 (PDT) Date: Tue, 2 Sep 2025 15:33:57 +0100 From: Johannes Weiner To: Wei Yang Cc: akpm@linux-foundation.org, vbabka@suse.cz, ziy@nvidia.com, linux-mm@kvack.org, vishal.moola@gmail.com, David Hildenbrand Subject: Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order Message-ID: <20250902143357.GA6654@cmpxchg.org> References: <20250902025807.11467-1-richard.weiyang@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250902025807.11467-1-richard.weiyang@gmail.com> X-Rspamd-Queue-Id: 967A74000D X-Stat-Signature: 4cui9ss7rqe8xqdz9fhix7qg9qef4fna X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1756823647-398517 X-HE-Meta: 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 EVucKbHz 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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 Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote: > We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy. > While if the order is less than start_pfn aligned order, we would get the > same pfn and do the same check again. > > Iterate from start_pfn aligned order to reduce duplicated work. > > Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com > Signed-off-by: Wei Yang > Cc: Johannes Weiner > Cc: Zi Yan > Cc: Vlastimil Babka > Cc: David Hildenbrand > Signed-off-by: Andrew Morton > Reviewed-by: Zi Yan > > --- > v2: add comment on assignment of order > --- > mm/page_alloc.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 07d79ae557f8..5d9ceca869e5 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page, > /* Look for a buddy that straddles start_pfn */ > static unsigned long find_large_buddy(unsigned long start_pfn) > { > - int order = 0; > + /* > + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing > + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking > + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy, > + * the starting order does not matter. > + */ > + int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER; This should be __ffs(start_pfn) - 1, no? If you have the lowest bit set in the pfn, you should check the order-1 buddy to the left first. But ffs(1) is already 1, which means the loop will check order-2 next.