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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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