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 56C38C54EE9 for ; Wed, 7 Sep 2022 21:22:55 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 9F5A96B0072; Wed, 7 Sep 2022 17:22:54 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 9A4D18D0003; Wed, 7 Sep 2022 17:22:54 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 86D8C8D0002; Wed, 7 Sep 2022 17:22:54 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 77EE16B0072 for ; Wed, 7 Sep 2022 17:22:54 -0400 (EDT) Received: from smtpin25.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 393C480236 for ; Wed, 7 Sep 2022 21:22:54 +0000 (UTC) X-FDA: 79886564268.25.86B0846 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by imf05.hostedemail.com (Postfix) with ESMTP id A6D20100088 for ; Wed, 7 Sep 2022 21:22:53 +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 sin.source.kernel.org (Postfix) with ESMTPS id 2D411CE1DB0; Wed, 7 Sep 2022 21:22:49 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B8137C433D6; Wed, 7 Sep 2022 21:22:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1662585767; bh=gxMGpmoNCeRaJ1bjDJWBLm6Nn6Ybto9MKRxMg9yg0CM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=omGfrsh2M987hhabKX/032BRuB7di5tlViD/uI57wqu8oWbhfgtnlGmVMjaRwJCmx kVvQhLcqICUIPAxfihnqj3Mfg57B9aC5b3hSZyTig+15RyUEzu6SJw2gHPDJZm6YvL 6S8uwjZ7hZ2KxjsppNkkhbu6CrLiMkpu9JK0SLLg= Date: Wed, 7 Sep 2022 14:22:46 -0700 From: Andrew Morton To: Yang Shi Cc: david@redhat.com, peterx@redhat.com, kirill.shutemov@linux.intel.com, jhubbard@nvidia.com, jgg@nvidia.com, hughd@google.com, aneesh.kumar@linux.ibm.com, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [v2 PATCH 1/2] mm: gup: fix the fast GUP race against THP collapse Message-Id: <20220907142246.c7d6f55e1b56fba3e16d974b@linux-foundation.org> In-Reply-To: <20220907180144.555485-1-shy828301@gmail.com> References: <20220907180144.555485-1-shy828301@gmail.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=omGfrsh2; spf=pass (imf05.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1662585773; a=rsa-sha256; cv=none; b=Fy4gEcLBbttr0+ShNif0Dfu8kTMY43sn7wY77bANocO4FSoPj6XxJdPy7khv/F7kZlGMpv UhNcYICYCzilz91VGbnsnCZetpNkvU1825G7jCLuL2fRsg35TjJx34k/2EeaC/gDR04PLD 0AyH1uAgb+oFKf+qd6U/Ddz7GPJr6Ds= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1662585773; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=Kz3z3lwfdVPE74y6fLzsb26nhOpQAfqEnI1d1lNxVp8=; b=b6erqNFq/bZlJrh+F4CWehCVw70RuEE3RR25uCXsLwZ/RL9HThXxKUd5C7hlLvckxIqmDE NFFMrJ4gCODOQ1xpNSNqtKINZo4OoR+Mdx700TcMgbBZZSnJs1i0h3uTCGkhMZA3oz/NhO JQGNY2+UbvN+XA8KNWxNcZRt8MONryk= Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=omGfrsh2; spf=pass (imf05.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-Rspam-User: X-Rspamd-Queue-Id: A6D20100088 X-Rspamd-Server: rspam10 X-Stat-Signature: puc99wgxodbz8qt3uwx1got18k3fjf6o X-HE-Tag: 1662585773-764134 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, 7 Sep 2022 11:01:43 -0700 Yang Shi wrote: > Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm: > introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer > sufficient to handle concurrent GUP-fast in all cases, it only handles > traditional IPI-based GUP-fast correctly. On architectures that send > an IPI broadcast on TLB flush, it works as expected. But on the > architectures that do not use IPI to broadcast TLB flush, it may have > the below race: > > CPU A CPU B > THP collapse fast GUP > gup_pmd_range() <-- see valid pmd > gup_pte_range() <-- work on pte > pmdp_collapse_flush() <-- clear pmd and flush > __collapse_huge_page_isolate() > check page pinned <-- before GUP bump refcount > pin the page > check PTE <-- no change > __collapse_huge_page_copy() > copy data to huge page > ptep_clear() > install huge pmd for the huge page > return the stale page > discard the stale page > > The race could be fixed by checking whether PMD is changed or not after > taking the page pin in fast GUP, just like what it does for PTE. If the > PMD is changed it means there may be parallel THP collapse, so GUP > should back off. > > Also update the stale comment about serializing against fast GUP in > khugepaged. > > Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()") Is this not worth a -stable backport?