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 62D9DC19F2B for ; Thu, 28 Jul 2022 18:35:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229482AbiG1Sf4 (ORCPT ); Thu, 28 Jul 2022 14:35:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35958 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229806AbiG1Sfv (ORCPT ); Thu, 28 Jul 2022 14:35:51 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B921B6E8A7 for ; Thu, 28 Jul 2022 11:35:50 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 5419F61DD6 for ; Thu, 28 Jul 2022 18:35:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A5CD3C433D6; Thu, 28 Jul 2022 18:35:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1659033349; bh=sAeQglxhv6IbWiE03sd4KpePvBU9wCT9BfSmg9/ulQw=; h=Date:To:From:Subject:From; b=olJf3+viR8x0068CSOGaNv4tbhptkT+p/VGUJTTxOXoJvGOHIW+IBpcBNPCASGtDo ClqfngxFVIz3K6Zh8weyailyM+KGteXAvlWsT1bSKY2BitOB0X0P4YWChuqbBNmWVc sGlN4rcphET+zR66QhXZqTWS9c07eq0YE7/yYB60= Date: Thu, 28 Jul 2022 11:35:48 -0700 To: mm-commits@vger.kernel.org, willy@infradead.org, will@kernel.org, vbabka@suse.cz, tglx@linutronix.de, rppt@linux.ibm.com, pcc@google.com, ira.weiny@intel.com, corbet@lwn.net, catalin.marinas@arm.com, bigeasy@linutronix.de, fmdefrancesco@gmail.com, akpm@linux-foundation.org From: Andrew Morton Subject: + documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch added to mm-unstable branch Message-Id: <20220728183549.A5CD3C433D6@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The patch titled Subject: Documentation/mm: rrefer kmap_local_page() and avoid kmap() has been added to the -mm mm-unstable branch. Its filename is documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: "Fabio M. De Francesco" Subject: Documentation/mm: rrefer kmap_local_page() and avoid kmap() Date: Thu, 28 Jul 2022 17:48:42 +0200 The reasoning for converting kmap() to kmap_local_page() was questioned recently.[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) kmap() also requires global TLB invalidation when its pool wraps and it might block when the mapping space is fully utilized until a slot becomes available. Warn users to avoid the use of kmap() and instead use kmap_local_page(), by designing their code to map pages in the same context the mapping will be used. [1] https://lore.kernel.org/lkml/1891319.taCxCBeP46@opensuse/ Link: https://lkml.kernel.org/r/20220728154844.10874-6-fmdefrancesco@gmail.com Signed-off-by: Fabio M. De Francesco Suggested-by: Ira Weiny Cc: Matthew Wilcox (Oracle) Cc: Mike Rapoport Cc: Sebastian Andrzej Siewior Cc: Thomas Gleixner Cc: Catalin Marinas Cc: Jonathan Corbet Cc: Peter Collingbourne Cc: Vlastimil Babka Cc: Will Deacon Signed-off-by: Andrew Morton --- Documentation/mm/highmem.rst | 5 +++++ 1 file changed, 5 insertions(+) --- a/Documentation/mm/highmem.rst~documentation-mm-prefer-kmap_local_page-and-avoid-kmap +++ a/Documentation/mm/highmem.rst @@ -84,6 +84,11 @@ list shows them in order of preference o be absolutely sure to keep the use of the return address local to the thread which mapped it. + Most code can be designed to use thread local mappings. User should + therefore try to design their code to avoid the use of kmap() by mapping + pages in the same thread the address will be used and prefer + kmap_local_page(). + Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered because the map implementation is stack based. See kmap_local_page() kdocs _ Patches currently in -mm which might be from fmdefrancesco@gmail.com are highmem-remove-unneeded-spaces-in-kmap_local_page-kdocs.patch highmem-specify-that-kmap_local_page-is-callable-from-interrupts.patch documentation-mm-dont-kmap-pages-which-cant-come-from-highmem.patch documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page.patch documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch highmem-delete-a-sentence-from-kmap_local_page-kdocs.patch documentation-mm-add-details-about-kmap_local_page-and-preemption.patch