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);
>> + }
>> +}
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