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 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 7A10FC47080 for ; Tue, 1 Jun 2021 12:02:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4C75E613AE for ; Tue, 1 Jun 2021 12:02:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233657AbhFAMDz (ORCPT ); Tue, 1 Jun 2021 08:03:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:33318 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230308AbhFAMDp (ORCPT ); Tue, 1 Jun 2021 08:03:45 -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 4C42F60200; Tue, 1 Jun 2021 12:02:04 +0000 (UTC) Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] 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 1lo35t-004oFR-Vk; Tue, 01 Jun 2021 13:02:02 +0100 Date: Tue, 01 Jun 2021 13:02:00 +0100 Message-ID: <87sg2123pj.wl-maz@kernel.org> From: Marc Zyngier To: Quentin Perret Cc: will@kernel.org, james.morse@arm.com, alexandru.elisei@arm.com, catalin.marinas@arm.com, suzuki.poulose@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kernel-team@android.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/7] KVM: arm64: Move hyp_pool locking out of refcount helpers In-Reply-To: <20210527125134.2116404-2-qperret@google.com> References: <20210527125134.2116404-1-qperret@google.com> <20210527125134.2116404-2-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: 62.31.163.78 X-SA-Exim-Rcpt-To: qperret@google.com, will@kernel.org, james.morse@arm.com, alexandru.elisei@arm.com, catalin.marinas@arm.com, suzuki.poulose@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kernel-team@android.com, linux-kernel@vger.kernel.org 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 Thu, 27 May 2021 13:51:28 +0100, Quentin Perret wrote: > > The hyp_page refcount helpers currently rely on the hyp_pool lock for > serialization. However, this means the refcounts can't be changed from > the buddy allocator core as it already holds the lock, which means pages > have to go through odd transient states. > > For example, when a page is freed, its refcount is set to 0, and the > lock is transiently released before the page can be attached to a free > list in the buddy tree. This is currently harmless as the allocator > checks the list node of each page to see if it is available for > allocation or not, but it means the page refcount can't be trusted to > represent the state of the page even if the pool lock is held. > > In order to fix this, remove the pool locking from the refcount helpers, > and move all the logic to the buddy allocator. This will simplify the > removal of the list node from struct hyp_page in a later patch. Is there any chance some documentation could be added so that we have a record of what the locking boundaries are? Something along the line of what we have in arch/arm64/kvm/vgic/vgic.c, for example. Thanks, M. -- Without deviation from the norm, progress is not possible.