Re: [PATCH v4 2/2] virtio_ring: check desc == NULL when using indirect with packed

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

On Wed, Oct 20, 2021 at 07:07:43PM +0800, Xuan Zhuo wrote:
> On Wed, 20 Oct 2021 04:24:33 -0400, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Wed, Oct 20, 2021 at 10:19:46AM +0800, Xuan Zhuo wrote:
> > > On Tue, 19 Oct 2021 09:21:51 -0400, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > On Tue, Oct 19, 2021 at 07:52:35PM +0800, Xuan Zhuo wrote:
> > > > > When using indirect with packed, we don't check for allocation failures.
> > > > > This patch checks that and fall back on direct.
> > > > >
> > > > > Fixes: 1ce9e6055fa ("virtio_ring: introduce packed ring support")
> > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > > ---
> > > > >  drivers/virtio/virtio_ring.c | 13 ++++++++++---
> > > > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 91a46c4da87d..44a03b6e4dc4 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -1065,6 +1065,9 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > > > >
> > > > >  	head = vq->packed.next_avail_idx;
> > > > >  	desc = alloc_indirect_packed(total_sg, gfp);
> > > > > +	if (!desc)
> > > > > +		/* fall back on direct */
> > > >
> > > > this comment belongs in virtqueue_add_packed right after
> > > > return.
> > > >
> > > > > +		return -EAGAIN;
> > > > >
> > > >
> > > > -ENOMEM surely?
> > >
> > > virtqueue_add_indirect_packed() will return -ENOMEM when dma mmap fails, so we
> > > have to choose a different error code.
> > >
> > > Thanks.
> >
> > That's exactly the point.  Why would we want to recover from allocation
> > failure but not DMA map failure?
> 
> >From indirect to direct mode, there is no need to allocate memory, so if we
> encounter memory allocation failure, we can fall back from indirect to direct
> mode. Memory allocation failure is a temporary problem.
> 
> And if you encounter a dma mmap error, this situation should be notified to the
> upper layer.
> 
> Thanks.

Why did dma map fail?
A common reason is precisely that we are running
out of buffer space. Using direct which does not
need extra buffer space thus has a chance to work.


> >
> > > >
> > > > >  	if (unlikely(vq->vq.num_free < 1)) {
> > > > >  		pr_debug("Can't add buf len 1 - avail = 0\n");
> > > > > @@ -1176,6 +1179,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > > >  	unsigned int i, n, c, descs_used, err_idx;
> > > > >  	__le16 head_flags, flags;
> > > > >  	u16 head, id, prev, curr, avail_used_flags;
> > > > > +	int err;
> > > > >
> > > > >  	START_USE(vq);
> > > > >
> > > > > @@ -1191,9 +1195,12 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > > >
> > > > >  	BUG_ON(total_sg == 0);
> > > > >
> > > > > -	if (virtqueue_use_indirect(_vq, total_sg))
> > > > > -		return virtqueue_add_indirect_packed(vq, sgs, total_sg,
> > > > > -				out_sgs, in_sgs, data, gfp);
> > > > > +	if (virtqueue_use_indirect(_vq, total_sg)) {
> > > > > +		err = virtqueue_add_indirect_packed(vq, sgs, total_sg, out_sgs,
> > > > > +						    in_sgs, data, gfp);
> > > > > +		if (err != -EAGAIN)
> > > > > +			return err;
> > > > > +	}
> > > > >
> > > > >  	head = vq->packed.next_avail_idx;
> > > > >  	avail_used_flags = vq->packed.avail_used_flags;
> > > > > --
> > > > > 2.31.0
> > > >
> >

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