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SubjectRe: [PATCHv2 RFC 4/4] Revert "virtio: make add_buf return capacity remaining:
On Thu, Jun 02, 2011 at 06:43:25PM +0300, Michael S. Tsirkin wrote:
> This reverts commit 3c1b27d5043086a485f8526353ae9fe37bfa1065.
> The only user was virtio_net, and it switched to
> min_capacity instead.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

It turns out another place in virtio_net: receive
buf processing - relies on the old behaviour:

try_fill_recv:
do {
if (vi->mergeable_rx_bufs)
err = add_recvbuf_mergeable(vi, gfp);
else if (vi->big_packets)
err = add_recvbuf_big(vi, gfp);
else
err = add_recvbuf_small(vi, gfp);
oom = err == -ENOMEM;
if (err < 0)
break;
++vi->num;
} while (err > 0);
The point is to avoid allocating a buf if
the ring is out of space and we are sure
add_buf will fail.

It works well for mergeable buffers and for big
packets if we are not OOM. small packets and
oom will do extra get_page/put_page calls
(but maybe we don't care).

So this is RX, I intend to drop it from this patchset and focus on the
TX side for starters.

> ---
> drivers/virtio/virtio_ring.c | 2 +-
> include/linux/virtio.h | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 23422f1..a6c21eb 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -233,7 +233,7 @@ add_head:
> pr_debug("Added buffer head %i to %p\n", head, vq);
> END_USE(vq);
>
> - return vq->num_free;
> + return 0;
> }
> EXPORT_SYMBOL_GPL(virtqueue_add_buf_gfp);
>
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 209220d..63c4908 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -34,7 +34,7 @@ struct virtqueue {
> * in_num: the number of sg which are writable (after readable ones)
> * data: the token identifying the buffer.
> * gfp: how to do memory allocations (if necessary).
> - * Returns remaining capacity of queue (sg segments) or a negative error.
> + * Returns 0 on success or a negative error.
> * virtqueue_kick: update after add_buf
> * vq: the struct virtqueue
> * After one or more add_buf calls, invoke this to kick the other side.
> --
> 1.7.5.53.gc233e


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