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 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?

> >
> > >  	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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