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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 C83C2C43381 for ; Tue, 2 Apr 2019 05:18:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E8192075E for ; Tue, 2 Apr 2019 05:18:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729002AbfDBFSL (ORCPT ); Tue, 2 Apr 2019 01:18:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57358 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726913AbfDBFSK (ORCPT ); Tue, 2 Apr 2019 01:18:10 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A875E85365; Tue, 2 Apr 2019 05:18:10 +0000 (UTC) Received: from xz-x1 (dhcp-14-116.nay.redhat.com [10.66.14.116]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A14F760159; Tue, 2 Apr 2019 05:18:04 +0000 (UTC) Date: Tue, 2 Apr 2019 13:18:02 +0800 From: Peter Xu To: Alex Williamson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, eric.auger@redhat.com, cohuck@redhat.com Subject: Re: [PATCH] vfio/type1: Limit DMA mappings per container Message-ID: <20190402051802.GB11008@xz-x1> References: <155414977872.12780.13728555131525362206.stgit@gimli.home> <20190402024115.GA11008@xz-x1> <20190401223413.3783af5f@x1.home> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190401223413.3783af5f@x1.home> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 02 Apr 2019 05:18:10 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 01, 2019 at 10:34:13PM -0600, Alex Williamson wrote: > On Tue, 2 Apr 2019 10:41:15 +0800 > Peter Xu wrote: > > > On Mon, Apr 01, 2019 at 02:16:52PM -0600, Alex Williamson wrote: > > > > [...] > > > > > @@ -1081,8 +1088,14 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu, > > > goto out_unlock; > > > } > > > > > > + if (!atomic_add_unless(&iommu->dma_avail, -1, 0)) { > > > + ret = -ENOSPC; > > > + goto out_unlock; > > > + } > > > + > > > dma = kzalloc(sizeof(*dma), GFP_KERNEL); > > > if (!dma) { > > > + atomic_inc(&iommu->dma_avail); > > > > This should be the only special path to revert the change. Not sure > > whether this can be avoided by simply using atomic_read() or even > > READ_ONCE() (I feel like we don't need atomic ops with dma_avail > > because we've had the mutex but it of course it doesn't hurt...) to > > replace atomic_add_unless() above to check against zero then we do > > +1/-1 in vfio_[un]link_dma() only. But AFAICT this patch is correct. > > Thanks for the review, you're right, we're only twiddling this atomic > while holding the iommu->lock mutex, so it appears unnecessary. Since > we're within the mutex, I think we don't even need a READ_ONCE. We can > simple test it before alloc and decrement after. Am I missing something > that would specifically require READ_ONCE within our mutex critical > section? Thanks, I don't know very clear on this and I'd be glad to learn about that. My understanding is that [READ|WRITE]_ONCE() is the same as volatile mem operation and will make sure we don't keep variables in the registers. So if the mutex semantics can support that (say, a "*addr = val" following with a mutex_unlock will make sure "val" will definitely land into memory of "&addr") then I do think it's fine even without it (which corresponds to WRITE_ONCE(&addr, val) in this case). Thanks, -- Peter Xu