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.
> 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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