Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 19, 2018 at 06:11:31AM +0900, Tetsuo Handa wrote:
> Michael S. Tsirkin wrote:
> > On Thu, Jan 18, 2018 at 10:30:18PM +0900, Tetsuo Handa wrote:
> > > On 2018/01/18 1:44, Michael S. Tsirkin wrote:
> > > >> +static void add_one_sg(struct virtqueue *vq, unsigned long pfn, uint32_t len)
> > > >> +{
> > > >> +	struct scatterlist sg;
> > > >> +	unsigned int unused;
> > > >> +	int err;
> > > >> +
> > > >> +	sg_init_table(&sg, 1);
> > > >> +	sg_set_page(&sg, pfn_to_page(pfn), len, 0);
> > > >> +
> > > >> +	/* Detach all the used buffers from the vq */
> > > >> +	while (virtqueue_get_buf(vq, &unused))
> > > >> +		;
> > > >> +
> > > >> +	/*
> > > >> +	 * Since this is an optimization feature, losing a couple of free
> > > >> +	 * pages to report isn't important.
> > > >> We simply resturn
> > > > 
> > > > return
> > > > 
> > > >> without adding
> > > >> +	 * the page if the vq is full. We are adding one entry each time,
> > > >> +	 * which essentially results in no memory allocation, so the
> > > >> +	 * GFP_KERNEL flag below can be ignored.
> > > >> +	 */
> > > >> +	if (vq->num_free) {
> > > >> +		err = virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> > > > 
> > > > Should we kick here? At least when ring is close to
> > > > being full. Kick at half way full?
> > > > Otherwise it's unlikely ring will
> > > > ever be cleaned until we finish the scan.
> > > 
> > > Since this add_one_sg() is called between spin_lock_irqsave(&zone->lock, flags)
> > > and spin_unlock_irqrestore(&zone->lock, flags), it is not permitted to sleep.
> > 
> > kick takes a while sometimes but it doesn't sleep.
> 
> I don't know about virtio. But the purpose of kicking here is to wait for pending data
> to be flushed in order to increase vq->num_free, isn't it?

It isn't. It's to wake up device out of sleep to make it start
processing the pending data. If device isn't asleep, it's a nop.

> Then, doesn't waiting for
> pending data to be flushed involve sleeping? If yes, we can wait for completion of kick
> but we can't wait for completion of flush. Is pending data flushed without sleep?
> 
> > 
> > > And walk_free_mem_block() is not ready to handle resume.
> > > 
> > > By the way, specifying GFP_KERNEL here is confusing even though it is never used.
> > > walk_free_mem_block() says:
> > > 
> > >   * The callback itself must not sleep or perform any operations which would
> > >   * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC)
> > >   * or via any lock dependency. 
> > 
> > Yea, GFP_ATOMIC would do just as well. But I think any allocation
> > on this path would be problematic.
> > 
> > How about a flag to make all allocations fail?
> > 
> > E.g. 
> > 
> > #define GFP_FORBIDDEN (___GFP_DMA | ___GFP_HIGHMEM)
> > 
> > Still this is not a blocker, we can worry about this later.
> > 
> > 
> > > > 
> > > >> +		/*
> > > >> +		 * This is expected to never fail, because there is always an
> > > >> +		 * entry available on the vq.
> > > >> +		 */
> > > >> +		BUG_ON(err);
> > > >> +	}
> > > >> +}
> > 

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