From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages Date: Wed, 7 Nov 2012 16:11:46 -0800 Message-ID: <20121107161146.b99dc4a8.akpm@linux-foundation.org> References: <265aaff9a79f503672f0cdcdff204114b5b5ba5b.1352256088.git.aquini@redhat.com> <87625h3tl1.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <87625h3tl1.fsf@rustcorp.com.au> 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: Rusty Russell Cc: Rik van Riel , Rafael Aquini , "Michael S. Tsirkin" , Konrad Rzeszutek Wilk , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, Peter Zijlstra , Andi Kleen , Minchan Kim , "Paul E. McKenney" List-Id: virtualization@lists.linuxfoundation.org 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! > > + /* 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. > 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).