Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756765Ab2HNTaZ (ORCPT ); Tue, 14 Aug 2012 15:30:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56808 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755161Ab2HNTaX (ORCPT ); Tue, 14 Aug 2012 15:30:23 -0400 Date: Tue, 14 Aug 2012 22:31:09 +0300 From: "Michael S. Tsirkin" To: Rafael Aquini Cc: Rusty Russell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Rik van Riel , Mel Gorman , Andi Kleen , Andrew Morton , Konrad Rzeszutek Wilk , Minchan Kim Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages Message-ID: <20120814193109.GA28840@redhat.com> References: <20120813084123.GF14081@redhat.com> <87lihis5qi.fsf@rustcorp.com.au> <20120814083320.GA3597@redhat.com> <20120814184409.GC13338@t510.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120814184409.GC13338@t510.redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2839 Lines: 74 On Tue, Aug 14, 2012 at 03:44:09PM -0300, Rafael Aquini wrote: > On Tue, Aug 14, 2012 at 11:33:20AM +0300, Michael S. Tsirkin wrote: > > On Tue, Aug 14, 2012 at 09:29:49AM +0930, Rusty Russell wrote: > > > On Mon, 13 Aug 2012 11:41:23 +0300, "Michael S. Tsirkin" wrote: > > > > On Fri, Aug 10, 2012 at 02:55:15PM -0300, Rafael Aquini wrote: > > > > > +/* > > > > > + * Populate balloon_mapping->a_ops->freepage method to help compaction on > > > > > + * re-inserting an isolated page into the balloon page list. > > > > > + */ > > > > > +void virtballoon_putbackpage(struct page *page) > > > > > +{ > > > > > + spin_lock(&pages_lock); > > > > > + list_add(&page->lru, &vb_ptr->pages); > > > > > + spin_unlock(&pages_lock); > > > > > > > > Could the following race trigger: > > > > migration happens while module unloading is in progress, > > > > module goes away between here and when the function > > > > returns, then code for this function gets overwritten? > > > > If yes we need locking external to module to prevent this. > > > > Maybe add a spinlock to struct address_space? > > > > > > The balloon module cannot be unloaded until it has leaked all its pages, > > > so I think this is safe: > > > > > > static void remove_common(struct virtio_balloon *vb) > > > { > > > /* There might be pages left in the balloon: free them. */ > > > while (vb->num_pages) > > > leak_balloon(vb, vb->num_pages); > > > > > > Cheers, > > > Rusty. > > > > I know I meant something else. > > Let me lay this out: > > > > CPU1 executes: > > void virtballoon_putbackpage(struct page *page) > > { > > spin_lock(&pages_lock); > > list_add(&page->lru, &vb_ptr->pages); > > spin_unlock(&pages_lock); > > > > > > at this point CPU2 unloads module: > > leak_balloon > > ...... > > > > next CPU2 loads another module so code memory gets overwritten > > > > now CPU1 executes the next instruction: > > > > } > > > > which would normally return to function's caller, > > but it has been overwritten by CPU2 so we get corruption. > > > > No? > > At the point CPU2 is unloading the module, it will be kept looping at the > snippet Rusty pointed out because the isolation / migration steps do not mess > with 'vb->num_pages'. The driver will only unload after leaking the total amount > of balloon's inflated pages, which means (for this hypothetical case) CPU2 will > wait until CPU1 finishes the putaback procedure. > Yes but only until unlock finishes. The last return from function is not guarded and can be overwritten. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/