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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 4781CC76191 for ; Wed, 24 Jul 2019 18:26:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 21B4C21951 for ; Wed, 24 Jul 2019 18:26:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727985AbfGXS0F (ORCPT ); Wed, 24 Jul 2019 14:26:05 -0400 Received: from mail-qt1-f196.google.com ([209.85.160.196]:41525 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727234AbfGXS0F (ORCPT ); Wed, 24 Jul 2019 14:26:05 -0400 Received: by mail-qt1-f196.google.com with SMTP id d17so46387437qtj.8 for ; Wed, 24 Jul 2019 11:26:04 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=47jnRb/JRiGyeu7VRGBMaSdfmagEfI1RSsTJGA+iCS4=; b=MHV96tRzvK+IX65b9645Y7FYlpB/Xpb/inwPJysl4415b8uBO0sOR9FXJ83IgMPjDU SlnDFVE8viz1O6+yerClo1357+uHJaX36soqQoEb4yg6rMjLjfKjugXahpKp3qdffL3y /z2HiaApK5uMyxdivofvjFKHOYSn9XvG4A/lVbl6rL1n5nsLfddUvvA7mquOap64BXzJ YFLNUmbDpXfKlhNY6ecdxzijOIpjvXNC7qivXS15zduIJWBeIY3e1h0HrynctmLV5JHp /sRxDxz2npuXE4oeB+bnu6pn8LpajD4H0STEYqcSFq5LtutpzHkUjpnHdaVZkkijrKKp TIVg== X-Gm-Message-State: APjAAAXpG5SaK7vB533Sr7GdzDkYobXEjXZWmiGkHh/147IlyPlY+r82 FDdjLlzKUZBDS4eS0a/EMtQrMA== X-Google-Smtp-Source: APXvYqwzliSrA8epYeAr+o2o6Ky/lp4cNdeDzQCES2sBPCPVe5rhDvrrKAuGBvuFcp/RzeYu5u8tYg== X-Received: by 2002:ac8:303c:: with SMTP id f57mr59008112qte.294.1563992764155; Wed, 24 Jul 2019 11:26:04 -0700 (PDT) Received: from redhat.com (bzq-79-181-91-42.red.bezeqint.net. [79.181.91.42]) by smtp.gmail.com with ESMTPSA id d26sm20464872qkl.97.2019.07.24.11.25.57 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 24 Jul 2019 11:26:03 -0700 (PDT) Date: Wed, 24 Jul 2019 14:25:54 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: syzbot , aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org Subject: Re: WARNING in __mmdrop Message-ID: <20190724142533-mutt-send-email-mst@kernel.org> References: <20190723010156-mutt-send-email-mst@kernel.org> <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com> <20190723032800-mutt-send-email-mst@kernel.org> <20190723062221-mutt-send-email-mst@kernel.org> <9baa4214-67fd-7ad2-cbad-aadf90bbfc20@redhat.com> <20190723110219-mutt-send-email-mst@kernel.org> <20190724040238-mutt-send-email-mst@kernel.org> <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On Wed, Jul 24, 2019 at 06:08:05PM +0800, Jason Wang wrote: > > On 2019/7/24 下午4:05, Michael S. Tsirkin wrote: > > On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote: > > > On 2019/7/23 下午11:02, Michael S. Tsirkin wrote: > > > > On Tue, Jul 23, 2019 at 09:34:29PM +0800, Jason Wang wrote: > > > > > On 2019/7/23 下午6:27, Michael S. Tsirkin wrote: > > > > > > > Yes, since there could be multiple co-current invalidation requests. We need > > > > > > > count them to make sure we don't pin wrong pages. > > > > > > > > > > > > > > > > > > > > > > I also wonder about ordering. kvm has this: > > > > > > > > /* > > > > > > > > * Used to check for invalidations in progress, of the pfn that is > > > > > > > > * returned by pfn_to_pfn_prot below. > > > > > > > > */ > > > > > > > > mmu_seq = kvm->mmu_notifier_seq; > > > > > > > > /* > > > > > > > > * Ensure the read of mmu_notifier_seq isn't reordered with PTE reads in > > > > > > > > * gfn_to_pfn_prot() (which calls get_user_pages()), so that we don't > > > > > > > > * risk the page we get a reference to getting unmapped before we have a > > > > > > > > * chance to grab the mmu_lock without mmu_notifier_retry() noticing. > > > > > > > > * > > > > > > > > * This smp_rmb() pairs with the effective smp_wmb() of the combination > > > > > > > > * of the pte_unmap_unlock() after the PTE is zapped, and the > > > > > > > > * spin_lock() in kvm_mmu_notifier_invalidate_() before > > > > > > > > * mmu_notifier_seq is incremented. > > > > > > > > */ > > > > > > > > smp_rmb(); > > > > > > > > > > > > > > > > does this apply to us? Can't we use a seqlock instead so we do > > > > > > > > not need to worry? > > > > > > > I'm not familiar with kvm MMU internals, but we do everything under of > > > > > > > mmu_lock. > > > > > > > > > > > > > > Thanks > > > > > > I don't think this helps at all. > > > > > > > > > > > > There's no lock between checking the invalidate counter and > > > > > > get user pages fast within vhost_map_prefetch. So it's possible > > > > > > that get user pages fast reads PTEs speculatively before > > > > > > invalidate is read. > > > > > > > > > > > > -- > > > > > In vhost_map_prefetch() we do: > > > > > > > > > >         spin_lock(&vq->mmu_lock); > > > > > > > > > >         ... > > > > > > > > > >         err = -EFAULT; > > > > >         if (vq->invalidate_count) > > > > >                 goto err; > > > > > > > > > >         ... > > > > > > > > > >         npinned = __get_user_pages_fast(uaddr->uaddr, npages, > > > > >                                         uaddr->write, pages); > > > > > > > > > >         ... > > > > > > > > > >         spin_unlock(&vq->mmu_lock); > > > > > > > > > > Is this not sufficient? > > > > > > > > > > Thanks > > > > So what orders __get_user_pages_fast wrt invalidate_count read? > > > > > > So in invalidate_end() callback we have: > > > > > > spin_lock(&vq->mmu_lock); > > > --vq->invalidate_count; > > >         spin_unlock(&vq->mmu_lock); > > > > > > > > > So even PTE is read speculatively before reading invalidate_count (only in > > > the case of invalidate_count is zero). The spinlock has guaranteed that we > > > won't read any stale PTEs. > > > > > > Thanks > > I'm sorry I just do not get the argument. > > If you want to order two reads you need an smp_rmb > > or stronger between them executed on the same CPU. > > > > Executing any kind of barrier on another CPU > > will have no ordering effect on the 1st one. > > > > > > So if CPU1 runs the prefetch, and CPU2 runs invalidate > > callback, read of invalidate counter on CPU1 can bypass > > read of PTE on CPU1 unless there's a barrier > > in between, and nothing CPU2 does can affect that outcome. > > > > > > What did I miss? > > > It doesn't harm if PTE is read before invalidate_count, this is because: > > 1) This speculation is serialized with invalidate_range_end() because of the > spinlock > > 2) This speculation can only make effect when we read invalidate_count as > zero. > > 3) This means the speculation is done after the last invalidate_range_end() > and because of the spinlock, when we enter the critical section of spinlock > in prefetch, we can not see any stale PTE that was unmapped before. > > Am I wrong? > > Thanks OK I think you are right. Sorry it took me a while to figure out. -- MST 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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,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 829B6C76186 for ; Wed, 24 Jul 2019 18:26:18 +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 57E4C2190F for ; Wed, 24 Jul 2019 18:26:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="AoUuA8w2" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 57E4C2190F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=KHFW3AWiKk8Y5UkZzJSyY03PGRJR8hsmrnImcDCg2rI=; b=AoUuA8w2/i5CYf kHjiSp0lgvNThZx6Ip6ClLlTxqdLLEZYT044zI2eoevh2FZigTlm1RT74kC1xGC3sOV3bw7nNSJil zlGvnUroKPEEGonTjSJr0HvvYSjJm3NUy3SXS9e0zTtg17YfkLrIDueqv8JKfYwRlZ9GmpcIgGT40 8gghncVGyOwiwCtDIkQzIfpoVjP5sCksPF97c3Jr3rvfX34HfkXmHndmm8tmd1pCDTix76RbiwSTE FGQ9UAJpPpz5Nba/YGOJEOSxYCSA9661Q8sEoENnmDSWfhG3DVZHS+xNyEjf+9eMDKdlaioVmwXke iOfAPtfXpdhv0LkNB2Jg==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hqLxo-0004p4-Ck; Wed, 24 Jul 2019 18:26:08 +0000 Received: from mail-qt1-f196.google.com ([209.85.160.196]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1hqLxl-0004oe-7K for linux-arm-kernel@lists.infradead.org; Wed, 24 Jul 2019 18:26:06 +0000 Received: by mail-qt1-f196.google.com with SMTP id a15so46443674qtn.7 for ; Wed, 24 Jul 2019 11:26:04 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=47jnRb/JRiGyeu7VRGBMaSdfmagEfI1RSsTJGA+iCS4=; b=fId7saHs3P4rbwIQVlvroe4DORIqYxM+ruN+ihTu2+NB0o/99IPCkBHHzzUxlu5ns0 SqAxs6THMPq60iB34RGG7RSQ40DxwxH5CIJuq7ZS1gjXm7aW+YWSaJSNXjaOtKRZp+wk LCiF9Jkh3S0Ttd9I4zPpNIhbYpNgr289+6Ekp75qGACe4uETgWQrPHgSyQuTdLPWlL1p Wl2NgQu8l2vVZEz27Iu/AV7kMG0T7geMcSurEUURv3pSH1eyrZBuKQPnKnmlKuNjijEa 3XlsOq9hB5+7zgmW+SEoXu9cGr3y5pw4RElepxL0wO5GFsMLxNap2OgJhh1ZH+WCdU6K HUkA== X-Gm-Message-State: APjAAAWH/VVpIubBXOkz7rmLwA2/B4Zf1CY3uCQZK6zGdDrlA+TrfBA6 YdcUgQ9dTYO0tdspLrzybZjoZw== X-Google-Smtp-Source: APXvYqwzliSrA8epYeAr+o2o6Ky/lp4cNdeDzQCES2sBPCPVe5rhDvrrKAuGBvuFcp/RzeYu5u8tYg== X-Received: by 2002:ac8:303c:: with SMTP id f57mr59008112qte.294.1563992764155; Wed, 24 Jul 2019 11:26:04 -0700 (PDT) Received: from redhat.com (bzq-79-181-91-42.red.bezeqint.net. [79.181.91.42]) by smtp.gmail.com with ESMTPSA id d26sm20464872qkl.97.2019.07.24.11.25.57 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 24 Jul 2019 11:26:03 -0700 (PDT) Date: Wed, 24 Jul 2019 14:25:54 -0400 From: "Michael S. Tsirkin" To: Jason Wang Subject: Re: WARNING in __mmdrop Message-ID: <20190724142533-mutt-send-email-mst@kernel.org> References: <20190723010156-mutt-send-email-mst@kernel.org> <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com> <20190723032800-mutt-send-email-mst@kernel.org> <20190723062221-mutt-send-email-mst@kernel.org> <9baa4214-67fd-7ad2-cbad-aadf90bbfc20@redhat.com> <20190723110219-mutt-send-email-mst@kernel.org> <20190724040238-mutt-send-email-mst@kernel.org> <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190724_112605_263974_BFB2D703 X-CRM114-Status: GOOD ( 27.19 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mhocko@suse.com, peterz@infradead.org, ldv@altlinux.org, james.bottomley@hansenpartnership.com, linux-mm@kvack.org, namit@vmware.com, mingo@kernel.org, elena.reshetova@intel.com, keescook@chromium.org, aarcange@redhat.com, davem@davemloft.net, hch@infradead.org, christian@brauner.io, syzbot , syzkaller-bugs@googlegroups.com, jglisse@redhat.com, viro@zeniv.linux.org.uk, linux-arm-kernel@lists.infradead.org, wad@chromium.org, linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, luto@amacapital.net, ebiederm@xmission.com, akpm@linux-foundation.org, guro@fb.com Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org T24gV2VkLCBKdWwgMjQsIDIwMTkgYXQgMDY6MDg6MDVQTSArMDgwMCwgSmFzb24gV2FuZyB3cm90 ZToKPiAKPiBPbiAyMDE5LzcvMjQg5LiL5Y2INDowNSwgTWljaGFlbCBTLiBUc2lya2luIHdyb3Rl Ogo+ID4gT24gV2VkLCBKdWwgMjQsIDIwMTkgYXQgMTA6MTc6MTRBTSArMDgwMCwgSmFzb24gV2Fu 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