From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2D4A725C818 for ; Mon, 2 Feb 2026 15:58:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770047926; cv=none; b=Hf4xRYNRWZQJIq8IzPuDtDTLklD3Rn2gF7wikwKMGYbTOzSE0qJ6NAVFj90uidDihQY74we/k66WiHTfTzGW/J/Nrs5xoYANkFwclHICM6RhmnA0DzK/0xbEmGQLY2XWyN7gwp/QLuWS1Lkd7jN/pM3V/yy+IKOhjGXWH+10alw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770047926; c=relaxed/simple; bh=hHQLWMhahxCY/meKke54BXwjoj9QXEZAW+ce8KhMydU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Jr+XPBembo4MudN/m5rQSb3k7wXDpLIsr6wJoWGwyFWcsXA8dvzuCcgjNhYzx3Mf1IDKJvqFdXQh6FDjv6lh6uM0GIZlU/lvohFcGqlR7nrGEuVpBBqfCzPW5vHNUrAGfJlVZlicWZzL86FUjA8lCbfv9Y/JJUwZbb4tdSn+Exc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GLpVBg1L; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GLpVBg1L" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3EB3AC2BC86; Mon, 2 Feb 2026 15:58:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770047926; bh=hHQLWMhahxCY/meKke54BXwjoj9QXEZAW+ce8KhMydU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=GLpVBg1LZNKswwsnWQ3UZCODEH36kG7z6vpM+HA3uwJpty7LFXOuhG/bBlxA+qWjq OJ9gk7An4n/muz+UpHVrUUz6Vshc/hmar5VtMNZ92Ypg7741E3DPeVE6ehWjEwiN+o FgKDepxGEwDy2L9aUQQ+TDTdFC2uCNtuEXCyGv9TT8oCjZQcDD1bBub6HWvihJfpdg 3+BJsc6oBHIVdppqHz2QP42lMFE1eBXLiXD1FbvHq9ctFC3Y5E6A0jPXTY/RQpnJ9T ExCYq22XT4i2B/oZnBhBEbFwBgwJSgDhLjSKY4DSH17hWYExpfvhJ/ML9LHQ9/MzLo OQ7FbT18qQKzw== Received: from phl-compute-03.internal (phl-compute-03.internal [10.202.2.43]) by mailfauth.phl.internal (Postfix) with ESMTP id 66437F40069; Mon, 2 Feb 2026 10:58:44 -0500 (EST) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-03.internal (MEProxy); Mon, 02 Feb 2026 10:58:44 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgddujeektdeiucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujf gurhephffvvefufffkofgjfhgggfestdekredtredttdenucfhrhhomhepmfhirhihlhcu ufhhuhhtshgvmhgruhcuoehkrghssehkvghrnhgvlhdrohhrgheqnecuggftrfgrthhtvg hrnhephfdufeejhefhkedtuedvfeevjeffvdfhvedtudfgudffjeefieekleehvdetvdev necuvehluhhsthgvrhfuihiivgepudenucfrrghrrghmpehmrghilhhfrhhomhepkhhirh hilhhlodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdduieduudeivdeiheeh qddvkeeggeegjedvkedqkhgrsheppehkvghrnhgvlhdrohhrghesshhhuhhtvghmohhvrd hnrghmvgdpnhgspghrtghpthhtohepvdekpdhmohguvgepshhmthhpohhuthdprhgtphht thhopegrkhhpmheslhhinhhugidqfhhouhhnuggrthhiohhnrdhorhhgpdhrtghpthhtoh epmhhutghhuhhnrdhsohhngheslhhinhhugidruggvvhdprhgtphhtthhopegurghvihgu sehrvgguhhgrthdrtghomhdprhgtphhtthhopeifihhllhihsehinhhfrhgruggvrggurd horhhgpdhrtghpthhtohepuhhsrghmrggrrhhifheigedvsehgmhgrihhlrdgtohhmpdhr tghpthhtohepfhhvughlsehgohhoghhlvgdrtghomhdprhgtphhtthhopehoshgrlhhvrg guohhrsehsuhhsvgdruggvpdhrtghpthhtoheprhhpphhtsehkvghrnhgvlhdrohhrghdp rhgtphhtthhopehvsggrsghkrgesshhushgvrdgtii X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 2 Feb 2026 10:58:42 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, Kiryl Shutsemau Subject: [PATCHv6 15/17] mm: Remove the branch from compound_head() Date: Mon, 2 Feb 2026 15:56:31 +0000 Message-ID: <20260202155634.650837-16-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260202155634.650837-1-kas@kernel.org> References: <20260202155634.650837-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The compound_head() function is a hot path. For example, the zap path calls it for every leaf page table entry. Rewrite the helper function in a branchless manner to eliminate the risk of CPU branch misprediction. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- include/linux/page-flags.h | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 1aaa604f4b9b..16384cb6f962 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void) static __always_inline unsigned long _compound_head(const struct page *page) { unsigned long info = READ_ONCE(page->compound_info); + unsigned long mask; + + if (!compound_info_has_mask()) { + /* Bit 0 encodes PageTail() */ + if (info & 1) + return info - 1; - /* Bit 0 encodes PageTail() */ - if (!(info & 1)) return (unsigned long)page; - - /* - * If compound_info_has_mask() is false, the rest of compound_info is - * the pointer to the head page. - */ - if (!compound_info_has_mask()) - return info - 1; + } /* * If compoun_info_has_mask() is true the rest of the info encodes * the mask that converts the address of the tail page to the head page. * * No need to clear bit 0 in the mask as 'page' always has it clear. + * + * Let's do it in a branchless manner. */ - return (unsigned long)page & info; + + /* Non-tail: -1UL, Tail: 0 */ + mask = (info & 1) - 1; + + /* Non-tail: -1UL, Tail: info */ + mask |= info; + + return (unsigned long)page & mask; } #define compound_head(page) ((typeof(page))_compound_head(page)) -- 2.51.2