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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 23DF3C433FE for ; Thu, 6 Oct 2022 14:15:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231947AbiJFOPJ (ORCPT ); Thu, 6 Oct 2022 10:15:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42796 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232399AbiJFOOp (ORCPT ); Thu, 6 Oct 2022 10:14:45 -0400 Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2257125EE; Thu, 6 Oct 2022 07:12:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1665065545; x=1696601545; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=M/d+8Ye9ckZlA4mM2GLYA6fMAWkJIf5vhmZPmWG1h1A=; b=KefSfVkhc7qeb8U1+vUU9yHQF1RZqH5HVpJVEuX18wnu1zMwRGCOF1fq 1TTKODF4TbH7SUO5zwF/NqmkYIVL7tx0E1RKc8ba7POSSv4oNnepL0Lx7 nP6Z9ij56bf6P1Q4QTc5G6EiOuRMV/NKgCygXkKsnZJeLmxE5yeoFfI+y U4fEuucHUYCAWnfy+MGvhhOAYNYD/iGlVjQVE7f5FJhswR+hpnBiFZyAW GKFV1w3pkjztzNXCjfEUEOyu2OidJCO0w8rGihEesahXtwj0gkOXhary9 XRE5d+LXfl6rkvLA33qHE/+6aEpBJxC7RaUGB39L7CoKZ8Y9HlCF/zJJB Q==; X-IronPort-AV: E=McAfee;i="6500,9779,10491"; a="329879453" X-IronPort-AV: E=Sophos;i="5.95,164,1661842800"; d="scan'208";a="329879453" Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Oct 2022 07:07:48 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6500,9779,10491"; a="627026853" X-IronPort-AV: E=Sophos;i="5.95,164,1661842800"; d="scan'208";a="627026853" Received: from liuzhao-optiplex-7080.sh.intel.com (HELO localhost) ([10.239.160.132]) by fmsmga007.fm.intel.com with ESMTP; 06 Oct 2022 07:07:44 -0700 Date: Thu, 6 Oct 2022 22:13:13 +0800 From: Zhao Liu To: Ira Weiny , "Fabio M . De Francesco" , Wei Liu Cc: "K . Y . Srinivasan" , Haiyang Zhang , Stephen Hemminger , Dexuan Cui , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H . Peter Anvin" , linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, Zhenyu Wang , Zhao Liu Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page() Message-ID: References: <20220928095640.626350-1-zhao1.liu@linux.intel.com> <21632541.EfDdHjke4D@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On Mon, Oct 03, 2022 at 01:24:51PM -0700, Ira Weiny wrote: > Date: Mon, 3 Oct 2022 13:24:51 -0700 > From: Ira Weiny > Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page() > > On Mon, Oct 03, 2022 at 07:43:49PM +0200, Fabio M. De Francesco wrote: > > On Wednesday, September 28, 2022 11:56:40 AM CEST Zhao Liu wrote: > > > From: Zhao Liu > > > > > > kmap() is being deprecated in favor of kmap_local_page()[1]. > > > > > > There are two main problems with kmap(): (1) It comes with an overhead as > > > mapping space is restricted and protected by a global lock for > > > synchronization and (2) it also requires global TLB invalidation when the > > > kmap's pool wraps and it might block when the mapping space is fully > > > utilized until a slot becomes available. > > > > > > With kmap_local_page() the mappings are per thread, CPU local, can take > > > page faults, and can be called from any context (including interrupts). > > > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, > > > the tasks can be preempted and, when they are scheduled to run again, the > > > kernel virtual addresses are restored and are still valid. > > > > > > In the fuction hyperv_init() of hyperv/hv_init.c, the mapping is used in a > > > single thread and is short live. So, in this case, it's safe to simply use > > > kmap_local_page() to create mapping, and this avoids the wasted cost of > > > kmap() for global synchronization. > > > > > > In addtion, the fuction hyperv_init() checks if kmap() fails by BUG_ON(). > > > From the original discussion[2], the BUG_ON() here is just used to > > > explicitly panic NULL pointer. So still keep the BUG_ON() in place to check > > > if kmap_local_page() fails. > > > > How might kmap_local_page() return invalid kernel addresses? > > > > I think that, if this function returns, the pointer is always a valid kernel > > address. Am I missing something? > > I think this is my mistake. I did not realize what Zhao was asking me the > other day and generally said to leave BUG_ON's alone and not change things. Thanks Fabio and Ira! This is my negligence, my consideration is still not thorough enough. I'll remove this BUG_ON(). > > In this case kmap_local_page() will not return NULL. It will BUG on its own if > it fails. > > > > > > Based on this consideration, memcpy_to_page() > > > is not selected here but only kmap_local_page() is used. > > > > I can't agree with you, if the premises are that kmap_local_page() might > > provide invalid addresses. > > I'm ok with the patch as is. But Fabio is correct and it may be worth removing > the check in a follow on patch. > Ira > > > Thanks, > > > > Fabio > > > > > Therefore, replace kmap() with kmap_local_page() in hyperv/hv_init.c. > > > > > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com > > > [2]: https://lore.kernel.org/lkml/20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2/ > > > > > > Suggested-by: Dave Hansen > > > Suggested-by: Ira Weiny > > > Suggested-by: Fabio M. De Francesco > > > Signed-off-by: Zhao Liu > > > > > > --- > > > Suggested by credits. > > > Dave: Referred to his comments about whether kmap() can fail and > > the > > > suggestion to keep BUG_ON() in place. > > > Ira: Referred to his task documentation and review comments about > > > keeping BUG_ON() for kmap_local_page(). > > > Fabio: Stole some of his boiler plate commit message. > > > --- > > > arch/x86/hyperv/hv_init.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > > > >