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 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 1C64BC4741F for ; Fri, 30 Oct 2020 13:07:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ACF50221FA for ; Fri, 30 Oct 2020 13:07:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="JKDGOHNW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726096AbgJ3NHB (ORCPT ); Fri, 30 Oct 2020 09:07:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49896 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725975AbgJ3NHB (ORCPT ); Fri, 30 Oct 2020 09:07:01 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 227DAC0613CF; Fri, 30 Oct 2020 06:06:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=rfcpHo3sYbHG9663+/oEKU5+yDacWVNSfCtmxqY30IA=; b=JKDGOHNWU/V8Y+2rs/k7xDcd+D ywFaOKdh66CG7CRxohaMqHaFLXYS9D8UEmY4cTPadxISCv3CHKX+79wCldI0LV8nLjJimNpRjUFot BbcFgkfMoOoVVqbSn6at/tUwkE3W4TeiBSkn85uGjZxEhlxcEUEn86bKSTG6W3x+/e6w76iTiIDMo +S7YG45wv9UKO0gFrKbYfO0n5SfTbF1pG2kSlc2FKZXNaKU1XoICbQHjohPVjgTGHFNPIe1Xmtc6u jm1U+Hxe5+XhivRe6Qrj/AgJuMiGDjrS/uMW8LgIirsxF3LkwsBxbZgQIkCutkdRTk0XP+Op+X95w kA9B9H+g==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kYU6t-0002Yw-HM; Fri, 30 Oct 2020 13:06:27 +0000 Date: Fri, 30 Oct 2020 13:06:27 +0000 From: Matthew Wilcox To: Thomas Gleixner Cc: LKML , linux-arch@vger.kernel.org, Linus Torvalds , Peter Zijlstra , Paul McKenney , David Airlie , Daniel Vetter , Ard Biesheuvel , Herbert Xu , Christoph Hellwig , Sebastian Andrzej Siewior , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Andrew Morton , linux-mm@kvack.org, x86@kernel.org, Vineet Gupta , linux-snps-arc@lists.infradead.org, Russell King , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, 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@lists.ozlabs.org, "David S. Miller" , sparclinux@vger.kernel.org, Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends Message-ID: <20201030130627.GI27442@casper.infradead.org> References: <20201029221806.189523375@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201029221806.189523375@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Thu, Oct 29, 2020 at 11:18:06PM +0100, Thomas Gleixner wrote: > This series provides kmap_local.* iomap_local variants which only disable > migration to keep the virtual mapping address stable accross preemption, > but do neither disable pagefaults nor preemption. The new functions can be > used in any context, but if used in atomic context the caller has to take > care of eventually disabling pagefaults. Could I ask for a CONFIG_KMAP_DEBUG which aliases all the kmap variants to vmap()? I think we currently have a problem in iov_iter on HIGHMEM configs: copy_page_to_iter() calls page_copy_sane() which checks: head = compound_head(page); if (likely(n <= v && v <= page_size(head))) return true; but then: void *kaddr = kmap_atomic(page); size_t wanted = copy_to_iter(kaddr + offset, bytes, i); kunmap_atomic(kaddr); so if offset to offset+bytes is larger than PAGE_SIZE, this is going to work for lowmem pages and fail miserably for highmem pages. I suggest vmap() because vmap has a PAGE_SIZE gap between each allocation. Alternatively if we could have a kmap_atomic_compound(), that would be awesome, but probably not realistic to implement. I've more or less resigned myself to having to map things one page at a time.