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=-4.0 required=3.0 tests=BAYES_00,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 3A171C07E9B for ; Mon, 19 Jul 2021 12:14:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1CF2061107 for ; Mon, 19 Jul 2021 12:14:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236830AbhGSLeN (ORCPT ); Mon, 19 Jul 2021 07:34:13 -0400 Received: from mail.kernel.org ([198.145.29.99]:35898 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236491AbhGSLeL (ORCPT ); Mon, 19 Jul 2021 07:34:11 -0400 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2AE0E60200; Mon, 19 Jul 2021 12:14:51 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1m5SAb-00EChJ-A3; Mon, 19 Jul 2021 13:14:49 +0100 Date: Mon, 19 Jul 2021 13:14:48 +0100 Message-ID: <87lf62jy9z.wl-maz@kernel.org> From: Marc Zyngier To: Quentin Perret Cc: james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com, tabba@google.com, dbrazdil@google.com, kernel-team@android.com, Yanan Wang Subject: Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings In-Reply-To: <20210719104735.3681732-4-qperret@google.com> References: <20210719104735.3681732-1-qperret@google.com> <20210719104735.3681732-4-qperret@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: qperret@google.com, james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com, tabba@google.com, dbrazdil@google.com, kernel-team@android.com, wangyanan55@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 19 Jul 2021 11:47:24 +0100, Quentin Perret wrote: > > The stage-2 map walkers currently return -EAGAIN when re-creating > identical mappings or only changing access permissions. This allows to > optimize mapping pages for concurrent (v)CPUs faulting on the same > page. > > While this works as expected when touching one page-table leaf at a > time, this can lead to difficult situations when mapping larger ranges. > Indeed, a large map operation can fail in the middle if an existing > mapping is found in the range, even if it has compatible attributes, > hence leaving only half of the range mapped. I'm curious of when this can happen. We normally map a single leaf at a time, and we don't have a way to map multiple leaves at once: we either use the VMA base size or try to upgrade it to a THP, but the result is always a single leaf entry. What changed? > To avoid having to deal with such failures in the caller, don't > interrupt the map operation when hitting existing PTEs, but make sure to > still return -EAGAIN so that user_mem_abort() can mark the page dirty > when needed. I don't follow you here: if you return -EAGAIN for a writable mapping, we don't account for the page to be dirty on the assumption that nothing has been mapped. But if there is a way to map more than a single entry and to get -EAGAIN at the same time, then we're bound to lose data on page eviction. Can you shed some light on this? Thanks, M. -- Without deviation from the norm, progress is not possible. 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=-4.0 required=3.0 tests=BAYES_00,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 0B1C3C07E9B for ; Mon, 19 Jul 2021 12:14:57 +0000 (UTC) Received: from mm01.cs.columbia.edu (mm01.cs.columbia.edu [128.59.11.253]) by mail.kernel.org (Postfix) with ESMTP id 7D7E46112D for ; Mon, 19 Jul 2021 12:14:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7D7E46112D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvmarm-bounces@lists.cs.columbia.edu Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 1013F4A193; Mon, 19 Jul 2021 08:14:56 -0400 (EDT) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ti8-m+xhHLOs; Mon, 19 Jul 2021 08:14:55 -0400 (EDT) Received: from mm01.cs.columbia.edu (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 0A65E4A1A7; Mon, 19 Jul 2021 08:14:55 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 60B5B4A193 for ; Mon, 19 Jul 2021 08:14:53 -0400 (EDT) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id R6EJL7Q1mUcP for ; Mon, 19 Jul 2021 08:14:52 -0400 (EDT) Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by mm01.cs.columbia.edu (Postfix) with ESMTPS id 627C34A126 for ; Mon, 19 Jul 2021 08:14:52 -0400 (EDT) Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2AE0E60200; Mon, 19 Jul 2021 12:14:51 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1m5SAb-00EChJ-A3; Mon, 19 Jul 2021 13:14:49 +0100 Date: Mon, 19 Jul 2021 13:14:48 +0100 Message-ID: <87lf62jy9z.wl-maz@kernel.org> From: Marc Zyngier To: Quentin Perret Subject: Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings In-Reply-To: <20210719104735.3681732-4-qperret@google.com> References: <20210719104735.3681732-1-qperret@google.com> <20210719104735.3681732-4-qperret@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: qperret@google.com, james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com, tabba@google.com, dbrazdil@google.com, kernel-team@android.com, wangyanan55@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Cc: kernel-team@android.com, qwandor@google.com, will@kernel.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Mon, 19 Jul 2021 11:47:24 +0100, Quentin Perret wrote: > > The stage-2 map walkers currently return -EAGAIN when re-creating > identical mappings or only changing access permissions. This allows to > optimize mapping pages for concurrent (v)CPUs faulting on the same > page. > > While this works as expected when touching one page-table leaf at a > time, this can lead to difficult situations when mapping larger ranges. > Indeed, a large map operation can fail in the middle if an existing > mapping is found in the range, even if it has compatible attributes, > hence leaving only half of the range mapped. I'm curious of when this can happen. We normally map a single leaf at a time, and we don't have a way to map multiple leaves at once: we either use the VMA base size or try to upgrade it to a THP, but the result is always a single leaf entry. What changed? > To avoid having to deal with such failures in the caller, don't > interrupt the map operation when hitting existing PTEs, but make sure to > still return -EAGAIN so that user_mem_abort() can mark the page dirty > when needed. I don't follow you here: if you return -EAGAIN for a writable mapping, we don't account for the page to be dirty on the assumption that nothing has been mapped. But if there is a way to map more than a single entry and to get -EAGAIN at the same time, then we're bound to lose data on page eviction. Can you shed some light on this? Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm 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=-5.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 1EAB8C07E9B for ; Mon, 19 Jul 2021 12:16:51 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D9C096113C for ; Mon, 19 Jul 2021 12:16:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D9C096113C Authentication-Results: mail.kernel.org; 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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m5SAh-009ZST-Ch; Mon, 19 Jul 2021 12:14:55 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m5SAd-009ZRw-Vy for linux-arm-kernel@lists.infradead.org; Mon, 19 Jul 2021 12:14:53 +0000 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2AE0E60200; Mon, 19 Jul 2021 12:14:51 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1m5SAb-00EChJ-A3; Mon, 19 Jul 2021 13:14:49 +0100 Date: Mon, 19 Jul 2021 13:14:48 +0100 Message-ID: <87lf62jy9z.wl-maz@kernel.org> From: Marc Zyngier To: Quentin Perret Cc: james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com, tabba@google.com, dbrazdil@google.com, kernel-team@android.com, Yanan Wang Subject: Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings In-Reply-To: <20210719104735.3681732-4-qperret@google.com> References: <20210719104735.3681732-1-qperret@google.com> <20210719104735.3681732-4-qperret@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: qperret@google.com, james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com, tabba@google.com, dbrazdil@google.com, kernel-team@android.com, wangyanan55@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210719_051452_086979_5493A03F X-CRM114-Status: GOOD ( 20.98 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, 19 Jul 2021 11:47:24 +0100, Quentin Perret wrote: > > The stage-2 map walkers currently return -EAGAIN when re-creating > identical mappings or only changing access permissions. This allows to > optimize mapping pages for concurrent (v)CPUs faulting on the same > page. > > While this works as expected when touching one page-table leaf at a > time, this can lead to difficult situations when mapping larger ranges. > Indeed, a large map operation can fail in the middle if an existing > mapping is found in the range, even if it has compatible attributes, > hence leaving only half of the range mapped. I'm curious of when this can happen. We normally map a single leaf at a time, and we don't have a way to map multiple leaves at once: we either use the VMA base size or try to upgrade it to a THP, but the result is always a single leaf entry. What changed? > To avoid having to deal with such failures in the caller, don't > interrupt the map operation when hitting existing PTEs, but make sure to > still return -EAGAIN so that user_mem_abort() can mark the page dirty > when needed. I don't follow you here: if you return -EAGAIN for a writable mapping, we don't account for the page to be dirty on the assumption that nothing has been mapped. But if there is a way to map more than a single entry and to get -EAGAIN at the same time, then we're bound to lose data on page eviction. Can you shed some light on this? Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel