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 205E046AF34; Fri, 27 Feb 2026 19:31:00 +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=1772220660; cv=none; b=YwFEsmRrCLaLj8S0gUxfzk/Rx1gQ4a/4Qqu1wl+4cTW+KGR18fEoJunLq1A7s9+hyecJ+wpqoa7P5RMdtbM6LHtFMP1eeOI5hFiNphp41ngwu0ln1BeEByBBSkqmLmseWBJpQviM4OjBzFYIMao8/Uyg5RvJdxKWj//Sj0AO034= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772220660; c=relaxed/simple; bh=c529vercbwstG5TFH+F+kzdVCfYFyALt69KitoOnUEM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=LnIRG9tRXjQxV1tniUASm6ylgO3aHA47ohA/kkpmSRU5ZYLu+jV/4lKM4pNeQdQkvi5B1ShqY2ZfBaPm/RoLcqMsbXojrSxzn8N021emXmK6BepXcQaN6qFu4EV446rBfEROX5LTjU/olxdXhtYx4vtHbBxeYt9nqyuIpETN6A8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BSuOLY8k; 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="BSuOLY8k" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 086CBC2BC86; Fri, 27 Feb 2026 19:30:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772220659; bh=c529vercbwstG5TFH+F+kzdVCfYFyALt69KitoOnUEM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=BSuOLY8kOSSPAKyZKQ18eO6ZqfbquV0qrDNmYrzRJw21d20h0kLgSOyXb2Khv+dKc Y1we41UsOstIsibh6KV/zLWu1fcIhRERsFWRuHuTspspJKNgxXJ2PxlRonD3Cblpux dtZnEv+a5YbZGTp1ihaErVnKp8B/0GD2puXCAIr+6hFlWA+4Jv5ijfNOhbWvZnvvn+ HMskYXWk28T85oTVKecqF6F+vKw6y7+H+Af/kBscjHWDUnJH+ExzNb+jXQNx86ZPqR KZwne2FH1GJJxkhv3TnnlEZUZF789C5QNhlyOynVoQH2PB/aPvxlMF+xhvt9pY4sGg YbcYnKJBCWUUQ== Received: from phl-compute-03.internal (phl-compute-03.internal [10.202.2.43]) by mailfauth.phl.internal (Postfix) with ESMTP id 1D72CF40069; Fri, 27 Feb 2026 14:30:58 -0500 (EST) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-03.internal (MEProxy); Fri, 27 Feb 2026 14:30:58 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgddvgeelkeegucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujf gurhephffvvefufffkofgjfhgggfestdekredtredttdenucfhrhhomhepfdfmihhrhihl ucfuhhhuthhsvghmrghuucdlofgvthgrmddfuceokhgrsheskhgvrhhnvghlrdhorhhgqe enucggtffrrghtthgvrhhnpefhudejfedvgeekffefvdekheekkeeuveeftdelheegteel gfefveevueekhfdtteenucevlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrih hlfhhrohhmpehkihhrihhllhdomhgvshhmthhprghuthhhphgvrhhsohhnrghlihhthidq udeiudduiedvieehhedqvdekgeeggeejvdekqdhkrghspeepkhgvrhhnvghlrdhorhhgse hshhhuthgvmhhovhdrnhgrmhgvpdhnsggprhgtphhtthhopedvledpmhhouggvpehsmhht phhouhhtpdhrtghpthhtoheprghkphhmsehlihhnuhigqdhfohhunhgurghtihhonhdroh hrghdprhgtphhtthhopehmuhgthhhunhdrshhonhhgsehlihhnuhigrdguvghvpdhrtghp thhtohepuggrvhhiugesrhgvughhrghtrdgtohhmpdhrtghpthhtohepfihilhhlhiesih hnfhhrrgguvggrugdrohhrghdprhgtphhtthhopehushgrmhgrrghrihhfieegvdesghhm rghilhdrtghomhdprhgtphhtthhopehfvhgulhesghhoohhglhgvrdgtohhmpdhrtghpth htohepohhsrghlvhgrughorhesshhushgvrdguvgdprhgtphhtthhopehrphhptheskhgv rhhnvghlrdhorhhgpdhrtghpthhtohepvhgsrggskhgrsehsuhhsvgdrtgii X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Fri, 27 Feb 2026 14:30:57 -0500 (EST) From: "Kiryl Shutsemau (Meta)" 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 , "David Hildenbrand (Arm)" Subject: [PATCHv7 16/18] mm: Remove the branch from compound_head() Date: Fri, 27 Feb 2026 19:30:17 +0000 Message-ID: <20260227193030.272078-16-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 From: Kiryl Shutsemau 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 Acked-by: David Hildenbrand (Arm) Reviewed-by: Vlastimil Babka --- 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 8716a27ef347..9ef2c3b28e07 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 compound_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