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=-5.8 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 E91F0C43332 for ; Wed, 17 Mar 2021 13:59:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B7EF664F6A for ; Wed, 17 Mar 2021 13:59:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231702AbhCQN7T (ORCPT ); Wed, 17 Mar 2021 09:59:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48482 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231470AbhCQN7F (ORCPT ); Wed, 17 Mar 2021 09:59:05 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 808ECC06174A for ; Wed, 17 Mar 2021 06:59:04 -0700 (PDT) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1615989542; 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=s7Dpc9wi5plmHVUqhEzB+naRL6Q83VO/uBFhukxo5j8=; b=C/eRE8ML00TQFE2haXfN9LQg/A/TsIYVa8qsf2dcK+5L/A8pLILENS3MyXjcsvaPuwTY2I zQkO/Wee7mobhmUdO13atpsDmB9O3HgD73/A8OzDh/UvOFutMubm9NqcVVZXOHZ5KzR45Y MP1J0/sMWEZsbFyxwFavyCS37diHrGO3eIiJrlzpGJnW2Y5Q1uU2IJhlBNEROdvV9GNWHN YA+11WZmt3iovKVdRIatomLhfQbDliDS0iqSsaAI6W9C+Dcdj2C8QMaEKGJ2Nf1ACqkego rG4pLjNopBsWx95+sCcEhtPr+T691r67ThRu2Vnh+d6/N27gKhKb2s1lM2tfPw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1615989542; 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=s7Dpc9wi5plmHVUqhEzB+naRL6Q83VO/uBFhukxo5j8=; b=E24NrR2WkMuti7VPjAor92J8iF/mcTaUuNwYHtLaK5kqJfOYo2jcTkZ3LC+ctcGmFNdplq nYhKr8uy4fBrPABQ== To: Ira Weiny , Christoph Hellwig Cc: linux-kernel@vger.kernel.org Subject: Re: kmap_local semantics In-Reply-To: <20210312153646.GE3014244@iweiny-DESK2.sc.intel.com> References: <20210312065413.GA29805@lst.de> <20210312153646.GE3014244@iweiny-DESK2.sc.intel.com> Date: Wed, 17 Mar 2021 14:59:01 +0100 Message-ID: <878s6lzxsq.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 12 2021 at 07:36, Ira Weiny wrote: > On Fri, Mar 12, 2021 at 07:54:13AM +0100, Christoph Hellwig wrote: >> So with the new kmap_local interface is it possible / advisable to >> use local kmaps over code that might schedule(), e.g. to wait for I/O? > > It is possible yes. "Advisable" I think so. Thomas knows better than I what > performance would be. The performance impact is only relevant for 32bit machines with HIGHMEM enabled. On 64bit kmap_local is a NOOP except when you enable CONFIG_DEBUG_KMAP_LOCAL there which then actually does the mapping (on x86 only for now) with guard maps between and that will do the 'drop map' on schedule out and restore on schedule in dance. But for regular 64bit there is absolutely _NO_ overhead. > FWIW I have been working on converting kmaps to kmap_local. Most of the > instances don't schedule AFAICT. The whole point of kmap_local is to be preemptible and does not have the nasty side effects of kmap_atomic, except for the 1 page per map and map nesting ordering requirements. > What I really don't want to see is any kmap'ings handed to another thread. I > am working hard to eliminate the use of kmap for that use. Is that going to be > a problem? kmap != kmap_local kmap_local mappings are thread local and can't be handed anywhere. It's documented. Thanks, tglx