Re: [PATCH net-next v5 1/4] vsock/virtio/vhost: read data from non-linear skb

On 01.08.2023 16:11, Paolo Abeni wrote:

On Sun, 2023-07-30 at 11:59 +0300, Arseniy Krasnov wrote:

quoted

This is preparation patch for MSG_ZEROCOPY support. It adds handling of
non-linear skbs by replacing direct calls of 'memcpy_to_msg()' with
'skb_copy_datagram_iter()'. Main advantage of the second one is that it
can handle paged part of the skb by using 'kmap()' on each page, but if
there are no pages in the skb, it behaves like simple copying to iov
iterator. This patch also adds new field to the control block of skb -
this value shows current offset in the skb to read next portion of data
(it doesn't matter linear it or not). Idea behind this field is that
'skb_copy_datagram_iter()' handles both types of skb internally - it
just needs an offset from which to copy data from the given skb. This
offset is incremented on each read from skb. This approach allows to
avoid special handling of non-linear skbs:
1) We can't call 'skb_pull()' on it, because it updates 'data' pointer.
2) We need to update 'data_len' also on each read from this skb.

It looks like the above sentence is a left-over from previous version
as, as this patch does not touch data_len. And I think it contradicts
the previous one, so it's a bit confusing.

Yes, seems I need to rephrase it in the next version. I meant that with
approach introduced in this patch we don't need to check that skb is
linear of non-linear after reading data from it. Because otherwise:
1) In case of linear skb we will need to call 'skb_pull()' after reading
   data, to update 'data' pointer.
2) In case of non-linear skb we will need to update 'data_len' field after
   reading data, as this field shows amount of data in fragged part.

quoted

Signed-off-by: Arseniy Krasnov <redacted>
---
 Changelog:
 v5(big patchset) -> v1:
  * Merge 'virtio_transport_common.c' and 'vhost/vsock.c' patches into
    this single patch.
  * Commit message update: grammar fix and remark that this patch is
    MSG_ZEROCOPY preparation.
  * Use 'min_t()' instead of comparison using '<>' operators.
 v1 -> v2:
  * R-b tag added.
 v3 -> v4:
  * R-b tag removed due to rebase:
    * Part for 'virtio_transport_stream_do_peek()' is changed.
    * Part for 'virtio_transport_seqpacket_do_peek()' is added.
  * Comments about sleep in 'memcpy_to_msg()' now describe sleep in
    'skb_copy_datagram_iter()'.

 drivers/vhost/vsock.c                   | 14 +++++++----
 include/linux/virtio_vsock.h            |  1 +
 net/vmw_vsock/virtio_transport_common.c | 32 +++++++++++++++----------
 3 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 817d377a3f36..8c917be32b5d 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c

@@ -114,6 +114,7 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		struct sk_buff *skb;
 		unsigned out, in;
 		size_t nbytes;
+		u32 frag_off;

IMHO 'offset' would be a better name for both the variable and the CB
field, as it can points both inside the skb frags, linear part or frag
list.

Ack

Otherwise LGTM, thanks!

Paolo

Thanks, Arseniy