From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id E6E16C54ED1 for ; Tue, 27 May 2025 19:28:55 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 819E06B0082; Tue, 27 May 2025 15:28:55 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7CA9C6B0099; Tue, 27 May 2025 15:28:55 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7073D6B009B; Tue, 27 May 2025 15:28:55 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 4AF846B0082 for ; Tue, 27 May 2025 15:28:55 -0400 (EDT) Received: from smtpin16.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id E9E13814A8 for ; Tue, 27 May 2025 19:28:54 +0000 (UTC) X-FDA: 83489675388.16.098ABC8 Received: from mout-p-102.mailbox.org (mout-p-102.mailbox.org [80.241.56.152]) by imf18.hostedemail.com (Postfix) with ESMTP id 14F4B1C0005 for ; Tue, 27 May 2025 19:28:52 +0000 (UTC) Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=pankajraghav.com header.s=MBO0001 header.b=iaV7rq0c; spf=pass (imf18.hostedemail.com: domain of kernel@pankajraghav.com designates 80.241.56.152 as permitted sender) smtp.mailfrom=kernel@pankajraghav.com; dmarc=pass (policy=quarantine) header.from=pankajraghav.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1748374133; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=mPtjj75LRfTYzMr7PmwGMbGTNi6uiMC9nL4Pt05dBys=; b=R2JX7e6nHJXr5QCeTJP4Y62a8ZZvfHWqBComgHm/SIAFR1nLG1qVmaPS2+l/vhIDavpjEL oIf6wHJwDYZsn219BhiQdKsa7byvjK8gmOqDvr9vGR5o6YdwSO94oTh2EDvIo5Gk78ugU2 18erskicUXqXV7Xr5pCAB3bB2Fvg3aQ= ARC-Authentication-Results: i=1; imf18.hostedemail.com; dkim=pass header.d=pankajraghav.com header.s=MBO0001 header.b=iaV7rq0c; spf=pass (imf18.hostedemail.com: domain of kernel@pankajraghav.com designates 80.241.56.152 as permitted sender) smtp.mailfrom=kernel@pankajraghav.com; dmarc=pass (policy=quarantine) header.from=pankajraghav.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1748374133; a=rsa-sha256; cv=none; b=VJzT/o2D5SzCNZFuTITbZCzNsfi/FgchnvnsuW+tIE/dITBcCj989GtO8xjjoAhbug7znm 7xqDgqZdnQF9VU5YsKGRWnjc/XlLrYChZqjNjFxWkJCiVOKjOwgmws+VzSN/YqZDKG6Fsk /PI+VYYTKhRUd7M0+uyioXMV7pV4WQI= Received: from smtp1.mailbox.org (smtp1.mailbox.org [IPv6:2001:67c:2050:b231:465::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mout-p-102.mailbox.org (Postfix) with ESMTPS id 4b6N3N3QXYz9vGt; Tue, 27 May 2025 21:28:48 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=pankajraghav.com; s=MBO0001; t=1748374128; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=mPtjj75LRfTYzMr7PmwGMbGTNi6uiMC9nL4Pt05dBys=; b=iaV7rq0c9sPviNFq8k/l6+ZI7uEV8a+jxbHrGmGr3AAFY6ozHA49ZP8mqDQ0Wz6PruzSU7 EW/9I2wxBhoaskTqLXrx8ObKhDqKUZcokjtPvJCbe+ZvkJJWhxb7g+/3APHCERvUIQdr0P jKpCD97ca7GHbFefbKnGzQBntKwW6JKPZqR3HoGYf7oC7yZ/9y29QApLKaGj2drvOpgy4m fHzOSSFx7vRM+2/nEjXJQLeFRijj61SmP/TSTvxQTaag/q8S+IJ7yolna3lq01psb2dSfX AJ4VcyVsBylnUm0dMVl9YiYeKTyLK9oTLL/w90cNBgs6suD4xClLwkIRf9Jrmw== Date: Tue, 27 May 2025 21:28:35 +0200 From: "Pankaj Raghav (Samsung)" To: Dave Hansen Cc: Pankaj Raghav , Suren Baghdasaryan , Ryan Roberts , Vlastimil Babka , Baolin Wang , Borislav Petkov , Ingo Molnar , "H . Peter Anvin" , Zi Yan , Mike Rapoport , Dave Hansen , Michal Hocko , David Hildenbrand , Lorenzo Stoakes , Andrew Morton , Thomas Gleixner , Nico Pache , Dev Jain , "Liam R . Howlett" , Jens Axboe , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, willy@infradead.org, x86@kernel.org, linux-fsdevel@vger.kernel.org, "Darrick J . Wong" , mcgrof@kernel.org, gost.dev@samsung.com, hch@lst.de Subject: Re: [RFC 2/3] mm: add STATIC_PMD_ZERO_PAGE config option Message-ID: References: <20250527050452.817674-1-p.raghav@samsung.com> <20250527050452.817674-3-p.raghav@samsung.com> <626be90e-fa54-4ae9-8cad-d3b7eb3e59f7@intel.com> <5dv5hsfvbdwyjlkxaeo2g43v6n4xe6ut7pjf6igrv7b25y2m5a@blllpcht5euu> <1c1f0ad7-8668-406b-9e4c-59ee52f816b3@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1c1f0ad7-8668-406b-9e4c-59ee52f816b3@intel.com> X-Rspam-User: X-Rspamd-Queue-Id: 14F4B1C0005 X-Rspamd-Server: rspam09 X-Stat-Signature: 91a33etph51enpxx6hk3wic97sgxcuc8 X-HE-Tag: 1748374132-128480 X-HE-Meta: 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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: > > You are right that if this config is disabled, the callers with NULL mm > > struct are guaranteed to fail, but we are not generating extra code > > because there are still users who want dynamic allocation. > > I'm pretty sure you're making the compiler generate unnecessary code. > Think of this: > > if (mm_get_huge_zero_folio(mm) > foo(); > else > bar(); > > With the static zero page, foo() is always called. But bar() is dead > code. The compiler doesn't know that, so it will generate both sides of > the if(). > Ahh, yeah you are right. I was thinking about the callee and not the caller. > If you can get the CONFIG_... option checks into the header, the > compiler can figure it out and not even generate the call to bar(). Got it. I will keep this in mind before sending the next version. > > Do you think it is better to have the code with inside an #ifdef instead > > of using the IS_ENABLED primitive? > It has nothing to do with an #ifdef versus IS_ENABLED(). It has to do > with the compiler having visibility into how mm_get_huge_zero_folio() > works enough to optimize its callers. I think something like this should give some visibility to the compiler: struct folio *huge_zero_folio __read_mostly; ... #if CONFIG_STATIC_PMD_ZERO_PAGE struct folio* mm_get_huge_zero_folio(...) { return READ_ONCE(huge_zero_folio); } #else struct folio* mm_get_huge_zero_folio(...) { } #endif But I am not sure here if the compiler can assume here the static huge_zero_folio variable will be non-NULL. It will be interesting to check that in the output. -- Pankaj