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 X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8439FC4346A for ; Mon, 21 Sep 2020 07:39:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 48CF320719 for ; Mon, 21 Sep 2020 07:39:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="gMaSuZ51"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="xr4r44cz" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726416AbgIUHjr (ORCPT ); Mon, 21 Sep 2020 03:39:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34154 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726244AbgIUHjo (ORCPT ); Mon, 21 Sep 2020 03:39:44 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0ECCAC061755; Mon, 21 Sep 2020 00:39:44 -0700 (PDT) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1600673980; 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=wvyp0RounB/4LwU0Wfnprorn1wbUFCaaQDFAPWZa1lY=; b=gMaSuZ51Lbc6QPkc00DoVI0ZeYFw1anwmwmCpCgA9mSAGscjICMP76VBc9c5LYYHJhtr1V szZo7+Ww9dytWz1dbrc8RA2obQDOH1QUnyfZvjgPqxmDJsiP27s/hsVMbOD0Zjwlv+ber/ qWGRVkhYTPW2tTmS4pYTCoYSiT9DI/ZTsnsQ80U+VCocs0lPSE8nq5758cGblIbk8SVdnx hPCPSLtdjkL9+573S5HhFCgk6FFXO/026nXOGnu4ti8sxk/fC+L2RQhONm/U5tVeg66C+O R0NLgGerMKEFgnb0uRm7Ot+c09xW+a47s/CiEhYERktxgUfSN56PQvvbENyzPg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1600673980; 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=wvyp0RounB/4LwU0Wfnprorn1wbUFCaaQDFAPWZa1lY=; b=xr4r44czYL0yv7MaHM0svfQR/cc+aXpFFvMpVd1kcucOlLs0NXid+7U1RRYcOhbAYMYtxi ganiHIU1DBLF2YCA== To: Linus Torvalds Cc: LKML , linux-arch , Paul McKenney , the arch/x86 maintainers , Sebastian Andrzej Siewior , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Will Deacon , Andrew Morton , Linux-MM , Russell King , Linux ARM , Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , David Airlie , Daniel Vetter , intel-gfx , dri-devel , Ard Biesheuvel , Herbert Xu , Vineet Gupta , "open list\:SYNOPSYS ARC ARCHITECTURE" , Arnd Bergmann , Guo Ren , linux-csky@vger.kernel.org, Michal Simek , Thomas Bogendoerfer , linux-mips@vger.kernel.org, Nick Hu , Greentime Hu , Vincent Chen , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev , "David S. Miller" , linux-sparc Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends In-Reply-To: References: <20200919091751.011116649@linutronix.de> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <87eemwcpnq.fsf@nanos.tec.linutronix.de> Date: Mon, 21 Sep 2020 09:39:39 +0200 Message-ID: <87a6xjd1dw.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Sun, Sep 20 2020 at 10:42, Linus Torvalds wrote: > On Sun, Sep 20, 2020 at 10:40 AM Thomas Gleixner wrote: >> >> I think the more obvious solution is to split the whole exercise: >> >> schedule() >> prepare_switch() >> unmap() >> >> switch_to() >> >> finish_switch() >> map() > > Yeah, that looks much easier to explain. Ack. So far so good, but Peter Z. just pointed out to me that I completely missed the fact that this cannot work. If a task is migrated to a different CPU then the mapping address will change which will explode in colourful ways. On RT kernels this works because we ping the task to the CPU via migrate_disable(). On a !RT kernel migrate_disable() maps to preempt_disable() which brings us back to square one. /me goes back to the drawing board. Thanks, tglx