linux-kernel - Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE

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Date:   Thu, 18 Jan 2018 22:30:18 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com
Subject: Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

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.
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. 

> 
>> +		/*
>> +		 * This is expected to never fail, because there is always an
>> +		 * entry available on the vq.
>> +		 */
>> +		BUG_ON(err);
>> +	}
>> +}