From: Rusty Russell Subject: Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages Date: Thu, 08 Nov 2012 09:32:18 +1030 Message-ID: <87625h3tl1.fsf@rustcorp.com.au> References: <265aaff9a79f503672f0cdcdff204114b5b5ba5b.1352256088.git.aquini@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <265aaff9a79f503672f0cdcdff204114b5b5ba5b.1352256088.git.aquini@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: linux-mm@kvack.org Cc: Rik van Riel , aquini@redhat.com, "Michael S. Tsirkin" , Konrad Rzeszutek Wilk , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Minchan Kim , Peter Zijlstra , Andi Kleen , Andrew Morton , "Paul E. McKenney" List-Id: virtualization@lists.linuxfoundation.org Rafael Aquini writes: > + * virtballoon_migratepage - perform the balloon page migration on behalf of > + * a compation thread. (called under page lock) > + if (!mutex_trylock(&vb->balloon_lock)) > + return -EAGAIN; Erk, OK... > + /* balloon's page migration 1st step -- inflate "newpage" */ > + spin_lock_irqsave(&vb_dev_info->pages_lock, flags); > + balloon_page_insert(newpage, mapping, &vb_dev_info->pages); > + vb_dev_info->isolated_pages--; > + spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags); > + vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE; > + set_page_pfns(vb->pfns, newpage); > + tell_host(vb, vb->inflate_vq); tell_host does wait_event(), so you can't call it under the page_lock. Right? You probably get away with it because current qemu will service you immediately. You could spin here in this case for the moment. There's a second call to tell_host(): > + /* > + * balloon's page migration 2nd step -- deflate "page" > + * > + * It's safe to delete page->lru here because this page is at > + * an isolated migration list, and this step is expected to happen here > + */ > + balloon_page_delete(page); > + vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE; > + set_page_pfns(vb->pfns, page); > + tell_host(vb, vb->deflate_vq); The first one can be delayed, the second one can be delayed if the host didn't ask for VIRTIO_BALLOON_F_MUST_TELL_HOST (qemu doesn't). We could implement a proper request queue for these, and return -EAGAIN if the queue fills. Though in practice, it's not important (it might help performance). Cheers, Rusty.