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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> > > > > ---
> > > > > 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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