Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754568Ab2KHAcc (ORCPT ); Wed, 7 Nov 2012 19:32:32 -0500 Received: from mx1.redhat.com ([209.132.183.28]:22813 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752828Ab2KHAcb (ORCPT ); Wed, 7 Nov 2012 19:32:31 -0500 Date: Wed, 7 Nov 2012 22:32:01 -0200 From: Rafael Aquini To: Andrew Morton Cc: Rusty Russell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, "Michael S. Tsirkin" , Rik van Riel , Mel Gorman , Andi Kleen , Konrad Rzeszutek Wilk , Minchan Kim , Peter Zijlstra , "Paul E. McKenney" Subject: Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages Message-ID: <20121108003159.GD10444@optiplex.redhat.com> References: <265aaff9a79f503672f0cdcdff204114b5b5ba5b.1352256088.git.aquini@redhat.com> <87625h3tl1.fsf@rustcorp.com.au> <20121107161146.b99dc4a8.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121107161146.b99dc4a8.akpm@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3166 Lines: 79 On Wed, Nov 07, 2012 at 04:11:46PM -0800, Andrew Morton wrote: > On Thu, 08 Nov 2012 09:32:18 +1030 > Rusty Russell wrote: > > > 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... > > Not really. As is almost always the case with a trylock, it needs a > comment explaining why we couldn't use the far superior mutex_lock(). > Data: this reader doesn't know! > That was just to alleviate balloon_lock contention if we're migrating pages concurrently with balloon_fill() or balloon_leak(), as it's easier to retry the page migration later (in the contended case). > > > + /* 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? > > Sleeping inside lock_page() is OK. More problematic is that GFP_KERNEL > allocation. iirc it _should_ be OK. Core VM uses trylock_page() and > the filesystems shouldn't be doing a synchronous lock_page() in the > pageout path. But I suspect it isn't a well-tested area. The locked page under migration is not contended by any other FS / core VM path, as it is already isolated by compaction to a private migration page list. OTOH, there's a chance of another parallel compaction thread hitting this page while scanning page blocks for isolation, but that path can be considered safe as it uses trylock_page() > > > 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). > -- 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/