From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx116.postini.com [74.125.245.116]) by kanga.kvack.org (Postfix) with SMTP id DCC106B0044 for ; Tue, 14 Aug 2012 23:51:50 -0400 (EDT) From: Rusty Russell Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages In-Reply-To: <20120814083320.GA3597@redhat.com> References: <20120813084123.GF14081@redhat.com> <87lihis5qi.fsf@rustcorp.com.au> <20120814083320.GA3597@redhat.com> Date: Wed, 15 Aug 2012 12:43:00 +0930 Message-ID: <87vcgkrgoz.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: "Michael S. Tsirkin" Cc: Rafael Aquini , 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 On Tue, 14 Aug 2012 11:33:20 +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. Actually, I have no idea. Where does virtballoon_putbackpage get called from? It's some weird mm thing, and I stay out of that mess. The vb thread is stopped before we spin checking vb->num_pages, so it's not touching pages; who would be calling this? Confused, Rusty. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org