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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 65BD6FA3742 for ; Thu, 27 Oct 2022 10:50:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id E43698E0002; Thu, 27 Oct 2022 06:50:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DF3218E0001; Thu, 27 Oct 2022 06:50:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CE2CD8E0002; Thu, 27 Oct 2022 06:50:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id BDBF18E0001 for ; Thu, 27 Oct 2022 06:50:49 -0400 (EDT) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 6EB99AB59E for ; Thu, 27 Oct 2022 10:50:49 +0000 (UTC) X-FDA: 80066411418.28.37C613E Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf21.hostedemail.com (Postfix) with ESMTP id 11E8F1C0016 for ; Thu, 27 Oct 2022 10:50:48 +0000 (UTC) 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 25CC16226B; Thu, 27 Oct 2022 10:50:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B5924C433C1; Thu, 27 Oct 2022 10:50:45 +0000 (UTC) Date: Thu, 27 Oct 2022 11:50:42 +0100 From: Catalin Marinas To: Anshuman Khandual Cc: Muchun Song , Wupeng Ma , Andrew Morton , Mike Kravetz , Muchun Song , Michal Hocko , Oscar Salvador , Linux Memory Management List , linux-kernel@vger.kernel.org Subject: Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap_pte Message-ID: References: <20221025014215.3466904-1-mawupeng1@huawei.com> <614E3E83-1EAB-4C39-AF9C-83C0CCF26218@linux.dev> <35dd51eb-c266-f221-298a-21309c17971a@arm.com> <3D6FDA43-A812-4907-B9C8-C2B25567DBBC@linux.dev> <3c545133-71aa-9a8d-8a13-09186c4fa767@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3c545133-71aa-9a8d-8a13-09186c4fa767@arm.com> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666867849; a=rsa-sha256; cv=none; b=vfogcH6PuYp7NOxTdemRXcgujsncndGnNhe1XXYEms8imHpnw0PXMBgt0LbqsnMPCKbiND VIAd6P7RlDApzOclS4k8ymlwKt1HH5oayd53ihEb5PFv/aifXvsRwx3x08x6n6lTnssTYu ++jwwsMRbtwQR8k0wkkmumZD8zG3xuc= ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=none; spf=pass (imf21.hostedemail.com: domain of cmarinas@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=cmarinas@kernel.org; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none) ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666867849; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=2VwmOn7WS5JJII9xCi/pjDGuotIKAjnyTa9a9uZGNPw=; b=FMKUiIQ8OH6ZLHWmfAn8FCQkOv9474E710MNJRlEBVAn7qG3ALwCyU7m4ZIsFBVM/DuzbK LCTHOizPHSMtvM1/RXc0cb6UNJY5AHUNo9SUByjLcluOIci6vB9a5qu6rtGv96mZWtRXkL HzECfyWB9OLcvvGJUKcYZD110OHsFYQ= X-Rspamd-Queue-Id: 11E8F1C0016 X-Rspam-User: Authentication-Results: imf21.hostedemail.com; dkim=none; spf=pass (imf21.hostedemail.com: domain of cmarinas@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=cmarinas@kernel.org; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none) X-Rspamd-Server: rspam04 X-Stat-Signature: iih9uhx9jrnphg7y9417go4z1dbu689d X-HE-Tag: 1666867848-128465 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Oct 26, 2022 at 02:06:00PM +0530, Anshuman Khandual wrote: > On 10/26/22 12:31, Muchun Song wrote: > >> On 10/25/22 12:06, Muchun Song wrote: > >>>> On Oct 25, 2022, at 09:42, Wupeng Ma wrote: > >>>> From: Ma Wupeng > >>>> > >>>> Commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with > >>>> each HugeTLB page") add vmemmap_remap_pte to remap the tail pages as > >>>> read-only to catch illegal write operation to the tail page. > >>>> > >>>> However this will lead to WARN_ON in arm64 in __check_racy_pte_update() > >>> > >>> Thanks for your finding this issue. > >>> > >>>> since this may lead to dirty state cleaned. This check is introduced by > >>>> commit 2f4b829c625e ("arm64: Add support for hardware updates of the > >>>> access and dirty pte bits") and the initial check is as follow: > >>>> > >>>> BUG_ON(pte_write(*ptep) && !pte_dirty(pte)); > >>>> > >>>> Since we do need to mark this pte as read-only to catch illegal write > >>>> operation to the tail pages, use set_pte to replace set_pte_at to bypass > >>>> this check. > >>> > >>> In theory, the waring does not affect anything since the tail vmemmap > >>> pages are supposed to be read-only. So, skipping this check for vmemmap > >> > >> Tails vmemmap pages are supposed to be read-only, in practice but their > >> backing pages do have pte_write() enabled. Otherwise the VM_WARN_ONCE() > >> warning would not have triggered. > > > > Right. > > > >> > >> VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte), > >> "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx", > >> __func__, pte_val(old_pte), pte_val(pte)); > >> > >> Also, is not it true that the pte being remapped into a different page > >> as read only, than what it had originally (which will be freed up) i.e > >> the PFN in 'old_pte' and 'pte' will be different. Hence is there still > > > > Right. > > > >> a possibility for a race condition even when the PFN changes ? > > > > Sorry, I didn't get this question. Did you mean the PTE is changed from > > new (pte) to the old one (old_pte) by the hardware because of the update > > of dirty bit when a concurrent write operation to the tail vmemmap page? > > No, but is not vmemmap_remap_pte() reuses walk->reuse_page for all remaining > tails pages ? Is not there a PFN change, along with access permission change > involved in this remapping process ? For the record, as we discussed offline, changing the output address (pfn) of a pte is not safe without break-before-make if at least one of the mappings was writeable. The caller (vmemmap_remap_pte()) would need to be fixed to first invalidate the pte and then write the new pte. I assume no other CPU accesses this part of the vmemmap while the pte is being remapped. -- Catalin